By Deane Morrison
U of M News Wire
Ethanol plants that use only the kernels of corn could slash their fossil fuel consumption and boost their energy efficiency by using the rest of the corn plant to generate heat and electricity, a new University of Minnesota study finds.
The study details the monetary and environmental costs and benefits of compacting the nonedible corn residue known as corn stover and burning it at ethanol plants to provide both heat and electricity. Currently, corn ethanol plants run largely on natural gas as a source of heat for processing the corn and on electricity from coal-fired power plants.
"Every unit of natural gas used emits 15 times as much carbon dioxide as burning a unit of biomass," said study author Vance Morey, a professor of bioproducts and biosystems engineering. "Burning coal emits about 25 times as much."
The study took into account the fossil fuels required to collect corn stover, turn it into round bales, grind and compact it and transport it to the ethanol plant or other facility that could burn it. Also accounted for was the cost of replacing nutrients lost to soil when corn stover is removed. Even given those costs, the researchers calculated that using compact corn stover would reduce life-cycle fossil CO2 emissions.
The key to corn stover lies in compacting the residue so that the semis carrying it to ethanol plants can make fewer trips.
The researchers studied a model system in which farmers produced large round bales of corn stover, which were stored at designated sites a mile or two from the fields. Then, using mobile units, the bales were first ground in tub grinders (similar to wood chippers). This produced a loose fluffy material that was immediately processed by a roll-press compactor.
The compactor produced slabs of corn stover with a density between six and 15 pounds per cubic foot.
"We're still working on compacting," Morey said. "We want to get a full 25-ton load on a semi. That's a typical maximum load."
The farm-to-ethanol-plant cost of delivering a ton of compacted corn stover came to $77 per ton; 30 percent of that expense was accounted for by replacing the nitrogen, phosphorus and potassium lost to the soil when the corn stover was removed from the fields. Moreover, refertilizing the soil produced 45 percent of the CO2 emissions associated with collecting and delivering the corn stover.
Producers must consider these nutrient replacement costs when deciding whether and for how much to sell their corn stover, he said.
The researchers will next turn their attention to the costs and benefits of using other biomass, such as switchgrass, mixed prairie grasses or alfalfa. Nutrients will have to be replaced for these crops, too, but the costs may be less since those plants need less fertilizer than corn. And alfalfa, a legume, can add nitrogen to soil. Morey said some nutrients are retained in ash from burned biomass and could be respread on fields.
The study was presented as a report to the University's Initiative for Renewable Energy and the Environment, which funded it.