Developing a Biofuels Rating System

Producing four-star biofuels may give producers a leg up when the United States develops a carbon cap-and-trade system. Methods to quantify greenhouse gas emissions and rate biofuels are being proposed and tested in an attempt to incorporate the multiple facets of cropping systems, conversion processes and industry and consumer needs.
By Susanne Retka Schill | July 20, 2007
All biofuels are not created equal. Renewable fuels have different carbon footprints, depending on the feedstock that's used to produce it, how that feedstock was grown, how far it was transported and how it was converted to ethanol. Before ethanol producers can join cap-and-trade programs or sell offset credits on the Chicago Climate Exchange, the hurdle to quantify and express greenhouse gas performance must be cleared. Although there are several systems and models being developed, EPM talked to researchers involved in two projects that are tackling the challenge from different angles. One is focused on a biofuels rating system, the other models a life cycle assessment of biofuels to provide a glimpse of what the future may hold for biofuels marketing.

Assigning one, two, three or four stars to biofuels based on how the feedstocks used were raised and the biofuels produced might seem a bit simplistic, but Alex Farrell, director of the University of California, Berkeley, Transportation Sustainability Research Center, favors that system from among the several that have been examined. He co-authored a study titled "Creating Markets for Green Biofuels: Measuring and Improving Environmental Performance," which was released in April 2007. "We've identified what we think are the key factors and ways of incorporating them into a system," Farrell says. That system would be useful for consumers, producers or the government if they wanted to regulate biofuels in some way, he adds.

Farrell suggests that star ratings be assigned using a Green Biofuels Index that incorporates the global warming intensity (GWI) rating associated with each batch of biofuel. The combined GWI of the feedstock biorefining process plus a more simplified feedstock rating would be computed into an index value. One star would be awarded for each 40 value units. "Feedstocks are very difficult to measure because they vary from county to county and even from farm to farm," Farrell says. He suggests there are capabilities already developed that can be adapted to a biofuels rating system to account for different cropping systems, such as the three-tier system developed by the USDA's Conservation Security Program. The tiers represent increasing levels of conservation practices.

Assigning a star and an index value number to biofuels could help blenders and consumers. "A company might use a value number in blending biofuels to hit its target," Farrell suggests. "But consumers might not want to learn what a 60 is, or an 83, so a star rating system is what the fuel could be marketed under." A company wanting to market a two-star biofuel could blend higher and lower index numbers to reach its target star rating.

Such a system could provide market incentives for producing greener biofuels, or provide a framework for a regulatory system if the government decided to go that route. At the very least, Farrell says their study can help improve the discussion around the environmental qualities of biofuels.

Life Cycle Assessments of Biofuels
Rating biofuels implies that one can quantify life cycle greenhouse gas (GHG) emissions. While many researchers have studied GHG emissions from corn production to the ethanol production process in the short term, a new study incorporates computer modeling to help quantify the greenhouse gas sinks and sources from several energy cropping systems and project affects over the long term. Sinks sequester carbon as opposed to sources, which emit greenhouse gases.

Biofuels advocates believe that renewable fuels have near-zero net emissions of greenhouse gases because the carbon is recycled, says Paul Adler, a research agronomist at the USDA Agricultural Research Service (ARS) in University Park, Pa. "However, growing the crops requires energy," he says. Adler cites the energy requirements for machinery and crop inputs, as well as the energy requirements for transporting feedstocks and the biofuels conversion itself. The picture gets even more complicated when considering farming practices, which affect the amount of carbon that the soil can sequester, and that greenhouse gases far more powerful than carbon dioxide are released during feedstock production. "We constructed a model which included the many factors contributing to life cycle greenhouse gas," Adler says. "This allowed us to compare the different energy crops and determine which ones reduce [greenhouse gases] the most."

The chart shows how a Green Biofuels Index might work for different cropping scenarios and biofuels technologies. GWI is global warming intensity. The formula shown in the middle column generates the number value. One star is awarded for each 40 value units.

He partnered with Bill Parton, senior research scientist at the Natural Resource Ecology Laboratory at Colorado State University in Fort Collins, Colo., and Stephen DelGrosso, a research soil scientist at the USDA ARS in Fort Collins, to design their unique analysis. Adler's work in cropping systems and trace greenhouse gas emissions from bioenergy cropping systems in Pennsylvania, and conducting life cycle assessments, was combined with the Colorado scientists' work in developing the ecological model called DAYCENT. DAYCENT is a process-based computer model, which simulates plant growth and the microbial processes in soil that lead to nitrous oxide and methane emissions.

They examined several energy crops and crop rotations: hybrid poplars, switchgrass, reed canary grass, corn-soybean rotations using conventional tillage, corn-soybean rotations with no-till methods, and corn-soybean-alfalfa rotations under both tillage systems. The cropping systems data was added to the current best estimates for the inputs and yields associated with cellulosic ethanol. When assessing corn, the model assumed that 50 percent of the corn stover was used to produce ethanol. Coproducts were assigned credits for energy and emissions because they displace competing products that require energy to make. "These numbers will evolve as the technology matures," says Adler. "It gives us a first look at how these systems will compare."

In the end, the winners in the greenhouse gas reduction comparisons aren't surprising, but the numerical values are interesting. Switchgrass and hybrid poplar energy crops transformed into biofuels provide the most greenhouse gas reductions when compared with gasoline and diesel, at about a 115 percent reduction, in the long term when soil carbon levels are at equilibrium and no longer sequestering additional amounts. Reed canary grass reduced greenhouse gas emissions by 85 percent. The different rotations and tillage systems for corn and soybean rotations reduced greenhouse gas emissions around 40 percent. These numbers compare with current analyses of the corn-to-ethanol production process showing a 20 percent greenhouse gas reduction, Adler says.

The greatest impact on reducing the amount of greenhouse gases associated with energy use by switching to biofuels came from eliminating the life cycle greenhouse gases from fossil fuel use, followed by the storage of carbon in the soil from perennial crops. People most often think of carbon when considering greenhouse gases, but agricultural systems release methane from the soil, which is 23 times more active than carbon dioxide as a greenhouse gas, and nitrous oxide, which is 300 times more powerful, Parton explains. Plus, the carbon sequestration effect of no-till or perennial crops has a relatively short-term positive effect on greenhouse gas emissions. "It's a positive for 20 years, then you reach a new equilibrium," Parton says.

These first attempts at biofuels comparisons are interesting, but more significant is the scientists' work to devise an assessment model that incorporates so many variables of the entire system. The report describing their methodology was published in the April 2007 Ecological Applications, a journal of the Ecological Society of America. Future work with the model will look at the potential impact of plowing up Conservation Reserve Program acres and planting the acreage to corn rotations, the impact of biofuels grown in different ecological regions and how climate change might impact biofuels.

Susanne Retka Schill is an Ethanol Producer Magazine staff writer. Reach her at or (701) 746-8385.