The Cellulosic Myth

- by Brian Tokar

[Excerpted from "Biofuels and the Global Food Crisis," in Fred Magdoff and Brian Tokar, eds., Agriculture and Food in Crisis: Conflict Resistance, and Renewal, New York: MONTHLY REVIEW PRESS, 2010.] 


Logging for biomass energy in Vermont

As concerns about agrofuels’ implications for food supplies and the environment have become more widespread, proponents have reaffirmed their claims that current technologies are merely a “stepping stone” to more sustainable biofuel production from the cellulose in grasses and trees, rather than from food starches and oilseed crops. They predict that the world will soon obtain increasing yields of liquid fuel extracted from prairie grasses, logging wastes and forest thinnings, as well as agricultural byproducts such as straw and corn stover (i.e., leaves and stems). The extraction of ethanol from these high-cellulose sources, however, is a complex, energy-consuming process involving many stages of enzymatic digestion and purification of breakdown products, followed by the fermentation of sugars into ethanol. Alternative processes, such as the high temperature gasification and distillation of cellulosic feedstocks—technically similar to the liquefaction of coal—have proven equally difficult to commercialize.

The most popular scenario for fuel extraction from cellulosic sources relies mainly on the use of wild or cultivated grasses, such as the varieties of switchgrass (Panicum virgatum) that briefly became synonymous with “cheap, abundant fuel” after President George W. Bush mentioned switchgrass in his 2006 State of the Union address. But harvesting grasses for fuel raises a host of new problems. Grass monocultures are highly dependent on nitrogen fertilizers and irrigation, while diverse grasslands, with healthy populations of leguminous plants, are highly productive and far better at sequestering carbon dioxide as soil organic matter. However, the use of mixed feedstocks in any industrial process significantly complicates the enterprise. Further, many grass species deemed suitable for agrofuel production are considered highly invasive. “[T]raits deemed ideal in a bioenergy crop,” reported one study, “are also commonly found among invasive species,” traits that include lack of known pests or diseases, high efficiency of water use and photosynthesis, rapid growth, and the ability to out-compete weeds in the spring.

In the United States, the most likely source of grass-based agrofuels is from grasslands now set aside under the Agriculture Department’s Conservation Reserve Program (CRP). In June 2006, representatives of twenty-two leading conservation and hunting advocacy groups wrote to Congress challenging proposals to grow fuel crops on conservation lands, citing the program’s remarkable success in reducing soil erosion, reducing weed pressure, and preserving wetlands. “Most at risk are the wildlife benefits of CRP,” the letter stated, “which to a great extent are simply not compatible with frequent harvesting.” Unlike the periodic fire disturbances that are necessary to sustain prairie ecosystems, harvesting grasslands returns few nutrients to the soil, and harvesting equipment would likely prove far more disruptive to wildlife than the spread of wildfire.

The use of crop residues for fuel also raises serious questions, as these materials are essential for soil conservation and play an essential role in agronomic cycles. The decomposition of crop residues tilled back into the soil after harvest is necessary for the maintenance of soil health, while growers who practice “no till” cultivation rely on the same residues as a mulch and for protection against soil erosion. Collecting and separating corn stover from the grain would require redesigned, probably heavier, combines, adding to farmers’ costs and to soil compaction. A 2007 study by researchers at two Department of Energy laboratories concluded that a maximum of 30 percent of crop residues could be removed without increasing soil erosion and lessening soil organic matter.

Finally, the proposed thinning of forests and removal of dead trees and branches for fuel production would reduce carbon sequestration and also threaten wildlife habitats. The experience of biomass power plants in the U.S. suggests that harvesting wood for fuel inevitably increases logging, whether in forests or on plantations dedicated to fuel production. Thinning operations disturb the forest floor, accelerating the loss of soil carbon as CO 2 . The push for cellulosic agrofuels has served to justify the expansion of monoculture tree plantations, as well as the development of genetically engineered trees, most notably the varieties of fast-growing eucalyptus that have been modified to survive in cooler climates such as those found in the Southeastern U.S. The South Carolina-based Arborgen company has repeatedly cited the search for appropriate biofuel feedstocks as a rationale for its aggressive development of genetically engineered tree varieties, and agrofuel development has also become a leading rationale for commercializing the exotic new genetic interventions known as “synthetic biology.”