April - June 2014
Burning wood for energy ignites debate over carbon emissions
BY HEIDI LI
COLUMBIA - On an average weekday, 16 tractor-trailers drive to MU's combined heat and power plant and two or three to Columbia's Municipal Power Plant, each delivering 25 tons of wood chips.
To reduce reliance on coal, the MU Power Plant started burning wood chips in 2006, and the city followed suit in 2008, burning wood chips in the summer and winter when demand for electricity is highest.
MU also built a biomass boiler last year that burns only wood chips.
About 90 percent of the wood chips that go to both plants comes from sawmill residue within 100 miles of Columbia, said Hank Stelzer, associate professor of the MU School of Natural Resources. The other 10 percent comes from whole trees cut from forests within 50 miles of town, as well as residue from commercial harvests.
The Environmental Protection Agency lists biomass, including wood chips and other plant materials, as a type of renewable energy that can help eliminate greenhouse gases. There are questions, however, about whether burning whole trees from forests actually reduces carbon in the atmosphere.
Some people regard burning biomass as environmentally friendly because, unlike coal or oil, trees can regrow and recapture the carbon they release when other trees are burned. Others believe cutting and burning whole trees might not only reduce the size of sustainable forests but also increase the time it takes remaining trees to recapture carbon.
Carbon neutral?
As the major source of greenhouse gases, carbon emissions come mostly from electricity generation, according to the EPA. Thus, "carbon neutrality" has been widely acknowledged as a standard for renewable energies. "Carbon neutral" means a power plant produces zero net carbon emissions by either offsetting the amount of carbon it releases or buying carbon credits to make up the difference.
MU uses the campus carbon calculator designed by Clean Air- Cool Planet to track its carbon emissions every year, said Meredith Elbaum, a sustainability consultant in Boston who helps MU update its annual climate action plan.
Some people regard burning wood chips as a carbon-neutral approach because they think the carbon released from burning wood chips can be offset.
According to the calculator's user guide, previous versions of the calculator regarded biomass as carbon neutral. When trees die, they release carbon into the atmosphere. As other trees grow, that carbon will be recaptured by photosynthesis. Thus, the total amount of carbon in the cycle doesn't increase.
What's more, according the guide, the carbon released from burning fossil fuels has been kept in the ground for millions of years, whereas the carbon emitted from burning wood chips adds only small amounts of carbon to what already exists.
Some scientists, however, say that although wood is a renewable resource that can regrow and recapture carbon, it is not carbon neutral.
The current carbon calculator doesn't treat burning wood chips as carbon neutral, but it does calculate carbon emissions from wood separately from fossil fuels, said Anna Pautler, the former campus program associate for Clean Air-Cool Planet, in an email.
Trees are about half carbon, and when they are burned, the carbon is released into the atmosphere as carbon dioxide. Whether the carbon can be offset by the regrowth of trees depends on many factors, such as the type ofwood burned, the amount of wood harvested and so on, Pautler said.
Carbon emissions
Environmental experts also cast doubt on whether burning whole trees reduces carbon emissions.
One of the major concerns is that the regrowth of trees is not guaranteed and that it takes time, said Sasha Lyutse, policy analyst at the Natural Resources Defense Council. Based on the latest scientific studies from around the country, she said it can take from 35 to 100 years to recapture carbon.
Our lands can be huge "carbon sinks," which means they absorb carbon dioxide in the atmosphere and store it in forests and soils, Lyutse said. Cutting and burning trees to produce energy can damage forest carbon sinks, allowing more carbon to be released into the atmosphere.
According to an EPA report, forests offset about 15 percent of total U.S. greenhouse gas emissions in 2012. Lyutse said we need more forests to help offset carbon emissions. Cutting and burning whole trees for energy adds a lot of carbon to the air, Lyutse said, and some studies show that it can also disrupt forest soil, creating additional carbon emissions.
There are alternative sources of biomass fuel that contain less carbon and include wood waste, reclaimed wood and timber harvest residues such as tree tops and limbs, Lyutse said. About 90 percent of the wood chips that MU and the city use to produce electricity fall into this category.
Both the MU and city power plants buy their wood chips from Foster Bros. Wood Products in Auxvasse. Ten percent of the wood for both power plants comes directly from forests in central Missouri, and the company cuts only trees that are marked by a forester trained to improve timber stands, Foster Bros. Vice President Steve Foster said. The trees that are cut are unhealthy and would likely die before the forest is harvested again, Stelzer said.
Forests managed by the company are harvested about every 15 years, Stelzer said. On a typical acre of forest that is harvested, 40 percent of the wood is taken for traditional wood products, such as lumber, flooring and pallets. Another 20 percent of the wood is harvested as biomass for producing energy, and the other 20 percent is left to grow for the next harvest, Stelzer said.
"We don't clear the land. We harvest what needs to be harvested, so we leave it in intact forests," he said. "Because we want to be sustainable and come back in another 15 years."
The amount of energy available from wood chips is by weight only about half that of coal, so the new biomass boiler at the MU Power Plant has to burn a large amount of wood chips, Stelzer said. It now burns about 120,000 tons annually, Stelzer said.
The city power plant burns 9,932 tons of wood chips per year. One-third of Missouri's 45 million acres is forest, mostly oak and hickory trees. Given the amount of the wood chips cut from whole trees that goes to the MU Power Plant, about 800 acres of forests in mid-Missouri are sustainably harvested to provide biomass to the power plant every year, Stelzer said.
How boilers work
About 12.5 percent of the city plant's total carbon emissions came from burning wood last year, said Christian Johanningmeier, power production superintendent for the Columbia Water and Light Department. The city power plant produced 66,661 megawatt-hours, which was 5.6 percent of the city total energy usage last year, and burning wood chips contributed about 13.4 percent of its total generation, he said.
Every winter and summer, the city power plant burns coal, wood chips and natural gas in its boilers to supplement the bulk of electricity it buys from outside Columbia and to meet the city's power demand in peak seasons, Johanningmeier said.
The two solid boilers at the city power plant are like two fourstory- high metal boxes, in which coal and wood chips are burned. Each boiler is connected to a stoker, which pours solid fuels into the boiler fire. As the fuel burns, water inside the pipelines aligned on the walls begins to boil into steam.
The steam rises inside the boiler and is funneled toward a turbine that, in turn, is connected to a generator. Pressure from the steam spins the turbine, causing the generator to produce electricity. The boilers at the MU plant work the same way, except that MU's new biomass boiler burns only wood chips.
The MU plant is capable of producing electricity for the entire campus. Its Combined Heat and Power Plant is nearly twice as efficient as a typical plant that does not use its excess steam to heat and cool buildings, Campus Facilities spokeswoman Karlan Seville said.
While the biomass boiler burns only wood chips, four others burn a mix of mostly coal with some wood chips added. The plant also uses natural gas during summer months, when natural gas prices are lower.
"The biomass boiler we put in a year ago reduces our dependence on coal and will be key in helping achieve the campus' sustainability goal of reducing the plant's coal use by 75 percent by 2017," Seville said.
MU reduced its carbon emissions by 28 percent from 2008 to 2013, and Seville estimated most of that was the result of the new biomass boiler. Because the boiler hasn't been operating for the entire fiscal year, it's impossible to say exactly how much it has reduced emissions.
Policy conundrum
Policy incentives for burning wood for energy might also lead to an increase in carbon emissions.
Traditionally, the number of biopower plants, which burn only biomass, is affected by the forestry industry of an area, said Thomas Johnson, MU professor of agricultural and applied economics.
For example, a report by Biomass Magazine says that Minnesota has 11 biopower plants, while Missouri has none.
"Because Minnesota has more forestry resources, they tend to have more biopower plants," Johnson said. "In addition, there may be more government incentives in Minnesota than in other states."
Governments offer both financial and regulatory incentives, Johnson said.
If those incentives don't take into account the possible harm that burning wood can bring to forests, there's a risk that the demand for burning whole trees will grow, Lyutse said.
"You build the demand and create this incentive that will go beyond the potential (sustainable) case."
There are alternatives to burning whole trees. MU has tested corncobs and switchgrass, but it is sticking with wood chips for now.
"If the market opens for other products, we could consider those, too," Seville said.
The city power plant tried miscanthus pellets, but Johanningmeier said they didn't work out. Miscanthus is a highyielding grass.
"Miscanthus pellets turned into powder before they reached the stokers," Johanningmeier said.
The city stopped testing them because of the possibility that miscanthus might ignite other fuels in the stoker before they reach the boiler where they're supposed to burn.
The city began burning wood chips again June 3. Johanningmeier said it also plans to work with a local company, Enginuity Worldwide, to test corn residue as a fuel in August.
Supervising editor is Sara Shipley Hiles. She worked with Heidi Li on this story as part of her Health, Science and Environmental Writing class at the Missouri School of Journalism.