Could A Low-emission Perennial Grass Help The Cellulosic Ethanol Industry?

By Niina Heikkinen

Biofuel producers looking for a cellulosic feedstock that is both highly productive and has a relatively low environmental impact should cultivate the perennial grass miscanthus, a recent study suggests.

Researchers at the University of Illinois, Urbana-Champaign, used computer modeling to run side-by-side comparisons of the potential production costs and levels of greenhouse gas emissions of different cellulosic feedstocks.

“The interesting thing about these feedstocks, there is a lot of diversity in their environmental impacts, in the cost of producing them, and the land requirements,” said Madhu Khanna, an environmental economist and one of the co-authors of the research.

The model considered 30 years of weather data from 1980 to 2011 to account for a full range of possible variability. The projected yields were based on real-world growing conditions recorded in three counties in Illinois, Indiana and Alabama.

Of the different crops tested, miscanthus was the top performer on a number of different measures. Compared with corn stover and switch grass, miscanthus had the lowest average life-cycle greenhouse gas emissions, boasted the highest average yields and sequestered the most carbon into the soil.

Of the three cellulosic biofuels, corn stover had net positive emissions compared with both perennial grasses. Switch grass and miscanthus had net negative life-cycle greenhouse gas emissions due to the amount of carbon the grasses were able to sequester into the soil over time.

Miscanthus had emissions savings of 135 to 165 percent depending on the soil quality, compared with 59 to 95 percent savings for corn stover. Switch grass fell in the middle of the two, with greenhouse gas emissions savings ranging from 100 to 150 percent, depending on the soil quality.

On average, miscanthus yields were 28 percent higher than switch grass and five times higher than corn stover. The cost of producing miscanthus was also two-thirds less than that of switch grass.

Both switch grass and miscanthus had somewhat lower yields when they were grown on low-quality soil, but the difference in yields was made up for by the lower cost of converting poorer-quality land, according to Khanna.

“It turns out it’s much cheaper to grow, and there are greater economic incentives to grow these on low-quality land than on productive cropland,” she said.

An invasive species?

The perennial grasses also sequestered more carbon into the soil on low-quality land than in higher-quality soils, according to the researchers.

Additional research had also found that miscanthus, along with energy cane, had the highest yields of cellulosic ethanol per acre of farmland, at over 800 gallons per acre. The least efficient performers were stover and wheat straw; both had yields under 200 gallons per acre. Sugar cane, switch grass and corn had ethanol yields somewhere in the middle.

Despite the environmental and yield benefits of cultivating cellulosic feedstocks like miscanthus, there are still significant questions about the viability of large-scale production for biofuel use.

One concern is that miscanthus could prove to be invasive. The hardy plant’s ability to grow in less-than-ideal conditions also means it can potentially spread outside cultivated fields.

Mary White, a former leader of Ireland’s Green Party, liked to pose in a field of miscanthus. Photo courtesy of the Green Party of Ireland.

“It makes sense that the same traits that you find in a fast-growing, easy-to-propagate plant are the same traits in invasive species,” said Luke Flory, an assistant professor of invasive plant ecology at the University of Florida, Gainesville, who was not involved with the study.

Flory’s own research has found that while sterile varieties of miscanthus have a low risk of becoming invasive, the risks are largely unknown for fertile varieties. He said more on-the-ground studies were needed to understand how local conditions affect the growth of different types of biofuel crops.

Although sterile miscanthus carries a much lower risk of becoming invasive, it is also more difficult and expensive for farmers to grow because they are propagating the crop with rhizomes rather than seeds, he said.

For farmers, that means purchasing specialized equipment, potentially making them less likely to plant the more controllable variety of perennial grass. Flory recommended that researchers conduct more on-the-ground studies to test how different biofuel crops respond to local environmental and soil conditions.

He described the University of Illinois study as a “rigorous analysis of the potential greenhouse gas savings from miscanthus-based ethanol.”

“The substantial reduction in greenhouse gases described in this system are noteworthy but questions remain about the overall environmental effects of bioenergy crops,” Flory wrote in an email. “While the invasion risk of sterile miscanthus is quite low, the development of a fertile variety might significantly increase the probability of invasion into surrounding natural areas. Clearly, the question of invasion risk will have to be revisited if a fertile variety is developed and released for cultivation.”

A call for incentives and ‘credible’ RFS implementation

Even though plants like miscanthus would work well as a dedicated biofuel crop, the future success of the cellulosic biofuel industry as a whole remains uncertain.

“We know that one of the reasons we don’t see too much cellulosic ethanol is because it is a costlier compound compared to gasoline and ethanol,” Khanna said.

The added costs of cellulosic ethanol come from all parts of the biofuel supply chain, but one of the most expensive aspects of production is breaking down the cellulose, hemicellulose and lignin in the plants’ cell walls. In recent years, the cost of enzymes used to break down the organic matter has declined significantly, helping to make cellulosic biofuel production more feasible, Khanna said.

In order to help spur the cellulosic biofuel industry forward, Khanna recommended more financial incentives for both farmers to grow the crops and for blenders to mix more cellulosic ethanol into conventional fuel. Those incentives could come in the form of tax credits or a carbon price.

Researchers at Kansas State University have found that farmers were more likely to cultivate biofuel feedstocks if they were offered shorter contracts (two rather than eight years), they had flexibility in how they cultivate crops and there was cost-sharing built in to reduce risk. Farmers were also more likely to grow the new and unfamiliar feedstocks if the crops were more profitable than their current enterprises, according to Jason Bergtold, an associate professor in Kansas State University’s Department of Agricultural Economics.