By Joel Tallaksen
West Central Research and Outreach Center
University of Minnesota,
The demand for locally produced, cost effective, and environmentally sound energy sources has increased as energy costs have risen and people better understand the environmental and geopolitical issues surrounding fossil fuels. Several different agricultural by-products and residues are being examined to satisfy energy needs. Corn cobs are one alternative that would avoid many of the drawbacks of using crude oil and coal. During the energy crisis of the 1970s, corn cobs were investigated as one of several alternatives to fossil fuels. However, as the oil crisis eased, the economic need for alternative fuels faded and, thus, related research fell off over time.
Composed of cellulose, hemi-cellulose, and lignin, cobs can be converted to a number of liquid and natural gas-like fuels via thermochemical or biochemical means. In addition, biorefining can generate a number of useful compounds, including plastics and starches, using cob components. Cobs have around 7,369 BTUs per pound when combusted, which is comparable to most other agricultural residues. Estimates for enzymatic conversion of corn cobs to ethanol are that 70-90 gallons of ethanol can be produced per ton of corn cobs using current technology. It is likely that improvements in both conversion technologies and crop genetics could increase the energy output from corn cobs in the near future.
An advantage to using corn cobs as an energy and biorefinery feedstock is their relatively low nutrient level and slightly higher energy value, in comparison to corn stover as a whole or other crop residues. Corn cobs are lower in nitrogen and phosphorus than stover and, therefore, using only corn cobs versus stover limits the amount of nutrients and organic matter that are removed from fields. Low feedstock nitrogen levels are also helpful in the thermochemical conversion processes, which can emit nitrogen as nitrous oxide, an EPA regulated air pollutant. In addition, phosphorus and potassium remain in the ash or char products from thermo conversion and can be re-applied to the fields the cobs were removed from. In fields yielding 150 bushels, estimates are that 1500 lbs of cobs can be harvested compared to the roughly 3 tons of stover available.
Corn cobs are much easier to handle than many of the fibrous agricultural residues. Being mostly uniform with a rigid structure, the cob is well suited for use with conventional industrial material handling equipment. It is easily processed using several types of grinders and can be resized based on specific needs. In comparison, stovers and straws are significantly more difficult to convey, resize, store and compress.
Though few farmers today harvest cobs and almost all farms and homes are heated with fossil fuels, cobs along with other biofuels were commonly used as the sole source of home/farm heating fuel through the middle of the last century. As farms grew larger in the middle of the last century, collection of ear corn became impractical because of collection, transportation, storage, and processing issues. Corn cob collection slowly fell off as farm equipment was re-designed to improve efficiency and remove only the grain from the field. Therefore, a good starting point for examining cobs as a renewable energy source is investigating collection and transportation of the cob. A variety of options are being examined by equipment manufacturers and third party suppliers. Prototype testing of third party combine add-ons is ongoing by custom equipment manufacturers. The first of the these products should be entering larger scale production in the coming year.
Testing of harvesting equipment, yields, and logistics is critical to assess the overall feasibility of corn cobs in renewable energy systems. This field level data would give those interested in using cobs the background to develop solid estimates of their biomass needs, the associated costs and the risks in pursuing large scale project. The West Central Research and Outreach Center is developing a research projects to look at large-scale harvesting of cobs using this new equipment. By conducting field scale analysis, this potential new energy source can be better understood and applied to a number of existing and planned bioenergy plants.