To make biofuels, or not to make biofuels:That is the question.
By Marguerite Torrey
Corn cobs, corn stover, and other crop residues are being developed as biofuel feedstocks.
In the past five years or so, opinion regarding the manufacture of biofuels has swung between approval and disapproval, and back again. First-generation fuels in particular, which are made from materials that can also serve as human or animal food, were initially praised, but then castigated. Biofuels have also been criticized as sources of greenhouse gas emissions. Second- and third-generation fuels are being explored with cautious optimism, but progress in generating fuels from biomass such as algae, forest materials, municipal wastes, and so on has been slow.
Identification of environmentally sound feedstocks
A recent article in Science (325:270-271, 2009) presents a consensus on the beneficial properties of biofuels reached by researchers from the University of Minnesota (Minneapolis, USA), Princeton University (New Jersey, USA), Massachusetts Institute of Technology (Cambridge, USA), and the University of California-Berkeley. The article results from a year of conversations and debate among some of the nation's leading biofuel experts, many of whom have been sharp critics of corn-based ethanol. In a nutshell, David Tilman, Robert Socolow, Jonathan A. Foley, Jason Hill, Eric Larson, Lee Lynd, Stephen Pacala, John Reilly, Tim Searchinger, Chris Somerville, and Robert Williams conclude, "Biofuels ‘done right' have a bright future in solving our energy and environmental challenges."
To balance biofuel production, food security, and emissions reduction, the authors point out that the global biofuels industry must focus on five major sources of renewable biomass:
- Perennial plants grown on degraded lands abandoned from agricultural use
- Crop residues
- Sustainably harvested wood and forest residues
- Double crops and mixed cropping systems
- Municipal and industrial wastes
These sources can provide at least 500 million short tons per year of biofuel in the United States alone.
Crop residues as biofuel feedstock
In a timely coincidence, the Springer-Verlag journal Cellulose devotes its August 2009 issue (16:531-762) to technological advances in the conversion of the residue from growing corn, or stover (i.e., cobs, stalks, and leaves remaining after corn grain is harvested), to biofuel. The 18 articles cover such topics as methods of treating stover to release fermentable substances, analysis of composition and sources of variability, the effect of delignification processes on cellulose digestibility, and the availability of stover for biomass conversion.
Richard Hess, Kevin L. Kenney, and Christopher T. Wright, of the Idaho National Laboratory (Idaho Falls, USA), and Robert Perlack and Anthony Turhollow, of Oak Ridge National Laboratory (Tennessee, USA), present an analysis of the costs involved in producing, harvesting, and collecting stover as well as the storing, handling and transportation, receiving, queuing, and preprocessing steps necessary for conversion to biofuels (Cellulose 16:599-619, 2009). They identify three barriers to the availability of corn stover for biorefining: (i) feedstock availability (Fig. 1), (ii) supply system logistics (i.e., the processes, capital, and operating costs associated with getting the stover resource from its production location to the in-feed system of the conversion process at the biorefinery), and (iii) characteristics of the biomass material itself.
Systems do exist to supply corn stover to biorefining facilities, but according to Hess and colleagues, present-day feedstock logistics systems are not well suited to prepare stover physically for processing (e.g., densifying the materials to facilitate handling and storage, controlling water activity, and controlling oxidation and microbial degradation).
Hess and co-workers conclude that improvement in equipment efficiency, without technological change, will not allow cost performance goals to be achieved with the conventional corn supply system.
Corncobs for cellulosic ethanol and methane
POET LLC, formerly known as Broin Companies, has been among the leaders in the United States in the production of bioethanol from corn. More recently, the company, headquartered in Sioux Falls, South Dakota, also has been developing technologies for making ethanol from cellulosic materials, using corncobs as feedstock. At its pilot-scale plant in Scotland, South Dakota, POET can produce about 20,000 gallons (76,000 liters) of ethanol a year from the cobs supplied by 40 farmers living within about 20 miles (32 kilometers) of the plant. According to Matt Merritt of its Media Relations office, POET is currently concentrating its efforts entirely on cobs, but it may look at other feedstocks for cellulosic ethanol.
Not content with producing only cellulosic ethanol, POET recently installed an anaerobic digester at the Scotland plant to process liquid waste, resulting from breaking down corncobs, to produce methane, or natural gas. This will be burned at the plant to supply heat for various processes.
POET is spending $200 million at another of its facilities (Emmetsburg, Iowa, USA) to scale up its Scotland cellulosic-ethanol pilot plant. According to the Associated Press, POET expects to receive $100 million of that amount in federal and state aid to build the plant, called Project LIBERTY. The Emmetsburg plant is scheduled to come online in the fall of 2011 and produce 25 million gallons (94 million liters) of cellulosic ethanol annually.
Being able to sell corncobs for ethanol production will provide another revenue stream for corn farmers. POET spokesman Nathan Schock told the Associated Press on July 16 that the company has not yet figured out how much it will pay farmers, but it could be $30-$60 per short ton for corn stover. (In Iowa, an acre of corn yields about 1.5 short tons of corn stover, or 3.4 metric tons [MT] per hectare. Of that 1.5 short tons of stover per acre, corncobs constitute about 0.65 short tons/acre, according to POET [Fig. 2].)
Scott Weishaar, vice president of commercial development, said, "Agriculture equipment manufacturers are putting the final touches on a variety of harvesters that will be on the market soon." For all farmers and companies wanting to make ethanol from corncobs, questions still remain: Will the machines to pick up corncobs from the field cost more than the cobs bring in? Will farmers store the cobs until they are needed at the ethanol plant, or can they be stored onsite? And who is responsible for transporting the cobs, the farmer or the ethanol plant?
Partial answers to those questions exist: The US Department of Agriculture Farm Service Agency has a program called BCAP (Biomass Crop Assistance Program) that will provide matching funds of up to $45 per short ton for two years to help farmers defray those start-up costs. Initial assistance will be for the collection, harvest, storage, and transportation costs associated with the delivery of eligible materials. As for collection of corncobs, POET is asking farmers to pile them at the edge of their fields, where POET or one of its contractors will pick up the cobs.
POET has been testing prototype equipment for harvesting corncobs in fields. The photographs with this article were taken in Iowa in November 2008, and the company also ran tests near Harlingen, Texas, USA, during July 2009, as a lead-up to efforts to harvest 25,000 acres (10,000 hectares) in Iowa, South Dakota, and Texas later in the fall. The company website has several videos of prototypes of experimental corncob-harvesting equipment in action (www.projectliberty.com).
The wisdom of making first-generation biofuels from grains that can also be eaten by humans or animals has been widely questioned in the past few years. Other feedstocks, including corn stover in general and corncobs in particular, are a real possibility for the second- and third-generation manufacture of bioethanol. Much work remains to be accomplished, however, to make ethanol generated from crop residues, or wood and forest residues, commercially feasible.
inform Technical Projects Editor Marguerite Torrey can be reached at firstname.lastname@example.org .