There's a lot of interest in creating fuels that produce less carbon dioxide, such as ethanol. But in northern Minnesota, people are starting to use another one -- wood waste from logging.
This form of biofuel, which can produce both heat and electricity, is not completely carbon free, and it can only provide a small portion of needed electricity. But it may be a partial solution, and one that helps keep loggers in business.
At the wood yard of the Laurentian Energy Authority, a bobcat loads tons of wood chips into tractor trailers that supply the power plants in Virginia and Hibbing, which have been partially transformed to burn wood instead of coal.
"It's different, it's unique, kind of fun, at times frustrating," said Terry Leoni, the authority's general manager. "We're traditional coal burners, so this is a really new venture for two stodgy municipal utilities."
The idea came from a painful economic reality: Both cities were thinking about shutting down their district heating plants. The plants burned coal to produce steam to heat businesses and homes via underground pipes. In the process, they also produced a small amount of electricity. It's called combined heat and power.
As the cities didn't have money to replace aging pipes, they planned to shut the operations down.
After they learned about a 1994 state law that required Xcel Energy to provide some electricity produced with biomass, they signed a contract with Xcel to produce power using wood waste. The law was part of a deal that allowed Xcel to store spent fuel at its Prairie Island nuclear power plan.
So the utilities of Virginia and Hibbing invested in new boilers designed to burn wood chips. It's been quite a learning experience.
Doug Ganoe, environmental manager for Virginia Public Utilities, said coal-fired boilers may contribute to global warming, but at least the technological bugs had been worked out. The newer wood boiler has been giving him headaches.
"With the wood boiler, you've got variation in the wood supply, you've got variation in the type of wood," said Ganoe, whose job includes pollution control. "It requires continuous monitoring, much closer than the coal unit."
Ganoe and his colleagues tinkered with the process for six months. The boiler kept putting out more carbon monoxide than the state permit allowed. Finally, they obtained state permission to raise the pollution limit. Ganoe said the operation is now running smoothly.
Xcel Energy pays a premium for the electricity, since wood waste is more expensive than coal per unit of heat it produces. For one thing, it's expensive to truck the wood from the forest to the generating plant.
There aren't many places that are located close to a wood supply, and have a use for the excess steam produced in a combined heat-and-power plant, said Dennis Becker, an assistant professor at the University of Minnesota's Department of Forest Resources.
Becker warns that the potential for wood to replace coal in power plans nationwide is small. Less than 10 percent of the electricity used nationwide could come from wood, he said.
"And that's OK," Becker said. "We look at forest resources, and our forests will only do so much."
Forests produce lumber and paper, provide recreation spots and help keep rivers and lakes clean.
Becker said there's a benefit to having a market for branches and other low-value parts of trees, because it helps loggers stay in business. Removing fuels from the woods also reduces the risk of wildfires.
Becker has studied the global warming potential of two hypothetical electric plants -- one coal-fired, one wood. He found that the wood burner put out one-fourth of the carbon dioxide compared to the coal burner. Burning wood also emitted only slightly more carbon dioxide than letting it decay on the forest floor. But Becker said it is not carbon-neutral.
"I would call it low-carbon, I would call it carbon-friendly," he said. "I would call it a hell of a lot better than the alternative."
There are possible downsides to using wood waste for electricity. Minnesota's forest is young. Every time a stand of trees is chopped down, it keeps the forest in its infancy -- and young forests store less carbon than older forests do.
Scientists also are concerned about the long-term ecological effects of removing so much organic material from the woods.
Anthony D'Amato, a professor in the U of M's Department of Applied Forest Ecology and Silviculture, said altering forest nutrients may produce unforeseen results. He is launching a long-term study to pinpoint the changes that can cause.