Despite a slight odor, sludge left over from production in local paper mills could provide one green solution to coal.

Marta Muilenburg, a graduate research assistant in mechanical engineering, started compiling data in June on available paper and wood waste within a 70 mile radius of Iowa City. The survey is part of a larger effort to help the UI Power Plant move away from fossil fuels.

Although Iowa sees a large amount of biomass produced every year, the availability of material is not well documented.

“That is pretty much where you want to be,” UI mechanical engineering Assistant Professor Albert Ratner said, pointing to Iowa on a map representing biomass availability across the nation. “But it is a little fuzzier than that. There is a lot of material available, but a lot is already in use.”

Muilenburg’s work should help clarify the biomass available in the area.

For example, the Weyerhaeuser paper mill in Cedar Rapids has 62,000 wet tons of paper sludge available per year, enough to replace 30 percent of coal in the UI Power Plant and feed all of the gasifiers in the proposed Oakdale power plant.

The paper sludge is one of various other paper and wood products Muilenburg is compiling. Sludge is a brown-green cardboard-like material clumped into nuggets. It is made of spare paper trimmings left over from production and the end stage of recycled paper.

“You can only recycle the material so many times,” Muilenburg said. “Then it becomes sludge.”

The proposed Oakdale Renewable Energy Plant would use gasifiers, which apply high heat to the fuel in an environment without oxygen to break the substance into gases that are, in turn, burned for energy.

Unlike the oat hulls used in the UI Power Plant, which contain about one-fourth of the energy of coal, the sludge contains about half the energy of coal. This means the plant would have to burn less paper than oat hulls to get the same amount of energy.

However, sludge poses a challenge because it contains 53 percent water.

“When you burn sludge, you have to burn the water first,” Muilenburg said. “You are putting a lot of energy into the water instead of burning the paper. Right now we are trying to figure out how efficient it would be to dry the sludge pre-burning.”

Muilenburg is performing preliminary drying tests in her home’s oven — starting off with a 9-inch pound-cake tin and moving to bigger bowls.

“My roommates came home, and they were like, ‘What is that smell,’ ” Muilenburg said, wrinkling her nose.

Sludge is not free of emissions, officials said, but it is a good alternative to coal.

“The paper was once actually trees, which used CO2,” Ratner said. “So it isn’t like you are releasing CO2 that was once buried.”

The survey of data is part of other UI research, including the testing of seed corn and soybeans.

After Muilenburg runs the survey, UI graduate student James Ulstad will test the energy produced by the sludge.

“Our focus is on retrofitting the biomass test system for seed corn, but we plan to test the paper sludge as soon as possible, and this can be done during the retrofitting process,” he said.

UI officials hope to include more biomass in the future, working toward the goal of being coal-free.

“Something has to be done,” Muilenburg said. “Fossil fuels will run out eventually, and we need to find something else.”