A pilot project testing the feasibility of burning corn cobs with coal found that cobs are a viable, renewable fuel opportunity for Willmar Municipal Utilities, according to a recently completed study.
Corn cobs were burned with coal at Willmar’s power plant in 2009 without operational issues. The study said cobs will provide sufficient heat value to make the co-combustion process efficient, and that cobs can be acquired locally at a volume and price sufficient to make co-combustion desirable.
The study said burning cobs and coal is permitted by the Minnesota Pollution Control Agency. MPCA gave Willmar a temporary air quality permit to carry out the test, but a permanent permit will probably take a year to obtain, said Jon Folkedahl of Folkedahl Consulting Inc. of Willmar, one of three study participants.
“The MPCA is in favor of this and will grant permits to do it,’’ said Folkedahl.
“We’re waiting on the MPCA at the moment.’’
Officials have said generating power by burning biomass such as corn cobs could help Willmar and other utilities comply with the state mandate of providing 25 percent of energy needs from renewable sources by 2025.
Folkedahl suggested the corn cob project to the Willmar Utilities in December 2007. In January 2008, U.S. Rep. Collin Peterson, D-Minn., directed a $300,000 Department of Energy grant to Willmar to fund the project as part of a larger power generation study.
Folkedahl said a feasibility analysis determined cobs at 15 percent moisture will provide about 75 percent of the heat value of coal. The power plant burns an average of 142 tons of coal per day.
“If we wanted to replace 10 percent of that by heat input, we’d need to burn about 20 tons per day of cobs or about 7,000 tons per year,’’ Folkedahl said.
He said Kandiyohi County could theoretically produce about 75,000 tons of cobs per year, based on an average of 150,000 acres of planted corn per year and about half a ton of cobs per acre.
The cobs were harvested with a machine called the Vermeer Cob Caddy. It’s pulled by a combine. The stover falls onto a belt, a fan blows the leaves and stalks out the back and the cobs fall into a basket.
Between Oct. 30 and Nov. 17, 2008, 421 tons of cobs were harvested on 631 acres. The cobs were stored on the former Willmar Airport runway and weathered the winter pretty well, said Folkedahl.
Officials had planned to mix cobs with coal in the silo and send the mixture through the distribution chutes into the stokers and the boiler.
But plans changed after officials visited a power plant in Pella, Iowa, where a similar co-combustion test was under way. The cobs on the conveyor belt feeding three boilers moved off to one side and two of three boilers received no cobs.
“We decided to avoid that scenario and determined the best way was to mix the cobs right in the fire box,’’ said Folkdahl.
He said Ken Nash, power plant supervisor, and his crew did an excellent job of fabricating and installing equipment that fed the cobs from an unused hopper by means of screw augers and silo pipe to an injection device nicknamed “The Cob Rocket.’’
The device, powered by overfire air, shot the cobs through a 3-inch by 9-inch view port into the furnace.
“It worked out very well,’’ said Folkedahl. “Some of the cobs would bounce off the far wall 16 feet away. We learned it was relatively easy to control the rate of injection.’’
The cost of burning cobs compared with coal depends on a number of factors including cob moisture content. The study said 1 ton of dry cobs will produce 1 megawatt-hour of electricity, while 2 tons of cobs at 47 percent moisture are needed to produce the same amount of electricity.
The moisture content of the stored cobs peaked at 47 percent in the spring but declined to 30 percent by summer 2009.
Based on a delivery cost of $37.50 per ton and burning 1.25 tons of cobs at 12 percent moisture, the cost of 1 megawatt-hour of electricity was calculated at $45, or about the same price as burning coal, the study said.
That price cannot be achieved without support from the U.S. Department of Agriculture’s new Biomass Crop Assistance Program, which pays the producer half of the cost of the cobs; the utility pays the other half.
Bruce Gomm, Willmar Utilities general manager, said the program makes burning cobs more attractive.
“That would be really good if we can take advantage of that,’’ he said.