The debate continues as scrutiny increases over the sustainability of ethanol derived from crops developed for food production.
Concerns about net energy and greenhouse gas emissions, in addition to its effect on food and feed pricing are driving researchers at the Energy Biosciences Institute (EBI) at the University of Illinois and at the University of California Berkeley to find cropping options that will produce ethanol sustainably and without taking more of the land currently used for food and feed production.
Sugarcane and miscanthus top the list of bioenergy crops that could produce enough ethanol to replace the United States' use of petroleum and escape U.S. dependence on fossil fuels, said Stephen P. Long, deputy director of the EBI at the U of I and Gutsgell Professor of Crop Sciences in the College of Agricultural, Consumer and Environmental Sciences.
"As new technologies increase the efficiency and cost of ethanol production, possibilities are opening up to use a wider range of plants to create a larger renewable fuel supply," he said. "Instead of repurposing food and feed crops, we are looking for dedicated energy crops with high production and low inputs to develop a system that's environmentally and economically sustainable."
In an article in Science titled "Feedstocks for Lignocellulosic Biofuels," researchers discussed the best plants to use for producing bioenergy in the future, taking into consideration that 600 million hectares of land worldwide have fallen out of agricultural production, mostly in the last 100 years.
"We have to avoid further competition with land currently producing food and feed while increasing the economic and environmental viability of our country," Long said. "Lignocellulosic biofuel production will create employment opportunities in areas where land has dropped out of productive use."
In 2008, the world produced 87 gigaliters of liquid biofuels, equivalent to the amount of liquid fuel used that year by Germany, a country of 100 million people. Brazil and the United States produced 90 percent of this fuel supply from sugarcane and corn.
The study showed Brazil could increase its production of sugarcane ethanol considerably, up to 14 percent of expected global use by 2030.
"These results can be achieved in Brazil without impacting food production," he said. "In addition, it would create strong benefits in terms of greenhouse gas emissions, and generate 22 megawatt hours of renewable electrical energy from residues that cannot be converted to ethanol."
While sugarcane has the greatest potential to produce biomass in the near future, the opportunity to develop sugarcane ethanol in the United States is limited to Hawaii, Florida, Mississippi, Louisiana, Alabama and Texas.
Miscanthus, on the other hand, is a highly productive perennial grass that can grow on marginal or non-crop land. It's a close relative to sugarcane, but tolerates cold temperatures sufficiently enough to be grown throughout the Midwest and into Canada, Long said.
One of the best characteristics of miscanthus is its ability to produce high biomass yields. EBI researchers reported that 17 tons per acre can be achieved in many locations in the Midwest and East.
"Miscanthus produces 60 percent more dry biomass than our best corn crops, but unlike corn it does it without any nitrogen fertilizer," Long said. "Unlike corn, its yield is largely independent of soil quality, though it does have similar water requirements per unit land area."
The incentive to discover high-yielding biomass crops stems back to the Energy Independence and Security Act (EISA) mandated by Congress in 2007. EISA mandates that by 2022, the United States will produce 36 billion gallons of ethanol per year.
EISA has raised considerable concern about the amount of land that would be required to achieve these goals. Some projections suggest that the land needed would exceed current crop areas.
Long said an area of Miscanthus equivalent to 5.5 percent of the current 360 million acres of U.S. land used for grain, oil seed, pulse, fruit and fiber production, would be needed to meet these mandates.
This land requirement is about half of the area placed into the Conservation Reserve Program. An alternative perspective is to utilize these lands accordingly to plant feedstocks for lignocellulosic biofuels.
"Miscanthus is not the only opportunity in the United States," Long said. "In fact, it's not suitable for dry areas in our country, and much of the land west of the Missouri River. It's also not suitable to strongly sloped lands or rocky lands, where productive trees represent an important feedstock for cellulosic ethanol."
In dry areas, agaves, a group of plants historically grown for alcoholic beverages and coarse fibers, can produce much more biomass per unit land area than miscanthus, trees, corn and other grain crops. Long said these feedstocks represent an important opportunity for utilizing the dry and abandoned lands of southwestern states.
In addition, miscanes (miscanthus-sugarcane hybrids) are likely to produce high yields per acre in the southernmost United States. Woody biomass can also be harvested sustainably for lumber and paper, providing biofuel feedstock for some regions.
Sarah Davis, U of I Bioenergy Feedstocks Analyst in EBI and adjunct assistant professor in plant biology, said the greatest opportunity for lignocellulosic biofuel production is the diversity of available feedstock options.
"In any given location, crop feedstock choices can be based on the local environmental conditions, and species can be selected based on productivity and ecosystem services that may be unique to the region," Davis said. "New bioenergy crops that grow on otherwise unproductive lands may offer an economic opportunity for regions that lack other land-based resources."
The search for new bioenergy crops to help meet EISA continues, Long said. EBI researchers are currently studying Napier grass. Results indicate that it could be twice as productive as miscanthus in the southern United States, reducing land requirements even further.
"The challenge remains that industry will need to invest in new plants that will last 30 years," he added. "Without a stable policy environment, these investments won't be possible at the state and federal levels."
Davis said more public awareness is needed.
"We must optimize the efficiency with which we produce all resources and develop a culture that rewards efficiency instead of convenience," Davis said. "Integrating land management for multiple resource demands is a necessity that will require broader societal awareness about the connection between consumer choices, agronomic practices, scientific discovery, and policy decisions."
This research appeared in the Aug. 13 issue of Science. Researchers included Chris Somerville, Heather Youngs and Caroline Taylor of the Energy Biosciences Institute at the University of California Berkeley, and Sarah Davis and Steve Long of the Energy Biosciences Institute at the University of Illinois, located in the Institute for Genomic Biology.