Crop residues as feedstock for renewable fuels
By Marguerite Torrey
Nutrient removal and net costs weigh on decisions to use crop residues as biofuel feedstocks.
The US Department of Energy's Office of Biomass Programs is encouraging the use of crop residue as a feedstock for production of renewable fuels. However the residue is processed to produce energy, one thing is certain, according to Fabián Fernández, University of Illinois extension specialist in soil fertility and plant nutrition: "It needs to be taken out of the field."
The amount of corn stover (all above-ground corn plant material except the grain) that can be removed without adverse consequences to the soil's level of organic matter, to physical and chemical properties of the soil, or to successive crop yields is not immediately apparent, according to Fernández. "Removing grain means removing nutrients from the soil. The export of plant nutrients from a field when crop residue is being removed is also an important point to consider."
Fernández pointed out that removing only the corncobs may be a way to increase bioenergy feedstock production while minimizing the long-term effect of residue removal on soil productivity. Corncobs represent only 20% of the total residue produced in a corn field, and their density and moisture content are more consistent than other parts of the crop residue. Furthermore, they can be collected in one operation during grain harvest, and they are easier to store because they are less susceptible to decomposition.
"Ultimately, the amount of phosphorus and potassium present in corncobs is far less than that in stalks and leaves," Fernández added, "so removal of corncobs represents less removal of nutrients from the field."
Methods to calculate the amount of stover produced, the amount of stover being removed, nutrient content of stover, and the estimated value of stover are available at http://tinyurl.com/UIUCstover.
Agricultural economist Wallace Tyner and graduate student Matthew Erickson of Purdue University (West Lafayette, Indiana, USA) are also interested in the economics of using corncobs as a feedstock for biofuels. They are investigating the question a bit further downstream than the University of Illinois work, looking at what economic incentives will make farmers decide to harvest corncobs. They calculated that farmers would need to receive about $100 per dry ton from biofuels companies to persuade most to add a cob-collection operation during fall corn harvest.
To determine economic feasibility, Tyner and Erickson examined a wide range of assumptions for cob collection, including the cost of cob-harvest equipment, the amount of time harvest is slowed by adding a cob-collection operation, the volume of cobs that could be collected per acre, and other factors.
In addition to the $100 per ton price, Tyner and Erickson discovered that farmers would be most likely to collect cobs if they made up at least 20% of the corn stover and if rental charges for cob-collection wagons were half the standard $28,000 seasonal rate.
Cobs are collected in a motorized wagon hitched to the back of a combine. At the time corn grain is separated from plant stover by the combine, stover is blown into the wagon instead of onto the ground. The wagon then separates cobs from other stover, which is blown onto the field. Once the wagon is filled with cobs, it is emptied into a truck or a designated dumping area.
The researchers used Purdue's B-21 PC-LP Farm Plan Model, a computer program that determines the return on a specific farm operation from production and other entered data, to develop their results. Anonymous data from 55 farms that supplied cobs to a Minnesota ethanol company in 2008 were plugged into the model, representing 100,264 acres (40,575 hectares). The model projected that none of the 55 farms would begin collecting cobs if wagon rental was $28,000 and cobs brought $40 a ton. If farm operators received $100 a ton from biofuels producers, B-21 projected 22 of the 55 farms would collect cobs.
This study can be downloaded from www.purdue.edu/newsroom/general/2010/100831TynerCobs.html.
POET Biorefining, which is headquartered in Sioux Falls, South Dakota, USA, is presently scaling up operations at its Project Liberty plant in Emmetsburg, Iowa, USA (inform 19:740, 2008; 20:254-155, 2009; 21:618, 2010). There, POET is scheduled to begin construction of a 25 million gallons (95 million liters) per year cellulosic ethanol plant that will process baled corn cobs and light stover.