Burning wood for energy leads to debate over its carbon emissions

By Heidi Li

COLUMBIA, Mo. — On an average weekday, 16 tractor-trailers drive to the MU’s Combined Heat and Power plant and two to three drive to the Columbia Municipal Power Plant, each containing 25 tons of wood chips.

According to the EPA report (http://www.epa.gov/ghgreporting/ghgdata/index.html), the total carbon dioxide emissions from non-biogenic sources has decreased from 2010 to 2012, whereas that from biogenic sources has increased. Biogenic sources refer to energy sources based on plants or plant materials. This graphic shows an increase in the amount of preserved forests needed to capture the carbon emission from biogenic sources. Graphic by Heidi Li.

To reduce reliance on coal, the MU power plant started to burn wood chips in 2006 and the city power plant started in 2008. MU also built a its biomass boiler last year that burns only wood chips. Of all the wood chips sent to the both power plants, 90 percent comes from sawmill residues within 100 miles of the city, said Hank Stelzer, associate teaching professor of the School of Natural Resources at the University of Missouri. But the other 10 percent comes from whole trees cut from forests within 50 miles of Columbia, as well as commercial harvest residues, Stelzer said.

According to the EPA website, biomass, including wood chips and other plant materials, is a kind of renewable energy, which can help eliminate greenhouse gas in the atmosphere. However, questions have been raised about whether burning whole trees from forests reduces carbon emissions.

Some people regard burning biomass as environmentally friendly because, unlike coal or oil, trees can regrow and recapture the carbon they release when they are burned. While some suggest that cutting whole trees and burning them may not only downsize sustainable forests but also cost trees more time to recapture the carbon than expected.

Is burning wood chips carbon neutral?

As the major source of greenhouse gases, carbon emissions come mostly from electricity generation, according to the EPA website. Thus, “carbon neutrality” has been widely acknowledged as a standard for renewable energies.

“Carbon neutral” means zero net carbon emissions by either offsetting the amount of carbon released 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 based in Boston, Mass., who helps the university develop its annual climate action plan update.

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Wood chips are burned in MU power plant and the city power plant to produce electricity. This is a picture of the wood chips used in the city power plant. Photo by Heidi Li.

Some people may regard burning wood chips as carbon neutral because they think the carbon released from burning wood chips can be offset, compared to burning fossil fuels. According to the calculator’s user guide, the previous versions of the calculator regarded biomass as carbon neutral. When trees die, they release carbon into the atmosphere. As other trees grow, the 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 is adding only a little carbon to what already exists in the cycle. So when fossil fuels are burned, much carbon is created without enough plants or trees to offset it. Wood, however, can be regrown.

However, though wood is a renewable resource that can regrow and recapture carbon, it is not carbon neutral, some scientists say.

The current carbon calculator doesn’t treat burning wood chips as carbon neutral, while it calculates carbon emissions from wood separately from fossil fuels, said Anna Pautler,  the former campus program associate of Clean Air-Cool Planet, said in an email.

Trees are about half carbon, and when they are burned, the carbon is released into that atmosphere in the form of carbon dioxide. Whether the carbon can be offset by regrowth of trees depends on many factors, such as type of energy produced, types of wood used and how much wood that is harvested, and so on, Pautler said.

Burning whole trees for energy may increase carbon emissions

Environmental experts also cast doubt on whether burning whole trees really reduces carbon emissions.

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Wood chips are the main source of biomass used in both power plants in Columbia. Photo courtesy of the Campus Facilities of MU.

One of the major concerns is that the regrowth of trees is not guaranteed and 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 the period of time for recapturing carbon can be 35 to more than 100 years.

Our lands can be huge “carbon sinks,” which means they absorb carbon dioxide out of the atmosphere and store it safely in forests and soils. When trees are cut and burned to produce energy, we damage the existing forest carbon sink, yet with more carbon released into the atmosphere​, Lyutse said.

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, not less, to help offset our carbon emissions.

“We are actually expecting to see an expansion of forests,” she said.

Cutting and burning whole trees for energy not only adds a lot of carbon to the air, but some studies show that it can also disrupt forest soil, creating additional carbon emissions, Lyutse said.

There are alternative source of biomass that could be used as fuels. Those fuels contain lower level of carbon and include wood waste, reclaimed wood, and timber harvest residues like tree tops and limbs, Lyutse said. About 90 percent of the wood chips that both MU and the city use to produce electricity fall into this category.

Small fraction of wood chips comes directly from forests in Missouri

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MU biomass boiler burns only wood chips. Photo courtesy of the Campus Facilities of MU.

Both the university and the city power plants buy their wood chips from Foster Brothers Wood Products, Inc., a company based in Auxvasse. The company started to send wood chips to the MU power plant in 2012 and to the city power plant in 2013.

Ten percent of the wood for both power plants come directly from forests in Central Missouri, and the company cuts only trees that are marked by a trained forester using the Timber Stand Improvement technique, said Steve Foster, vice president of Foster Brothers Wood Products, Inc. The trees that are cut are of poor form or health and will likely die before the forest is harvested again, Stelzer said.

The forests that are 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 harvested 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.”

Given an equal weight of wood chips and coal, the amount of energy available from wood chips is only about half that of coal, so the new biomass boiler of the university power plant has to burn a large amount of wood chips, Stelzer said.

The MU power plant now burns about 120,000 tons of wood chips annually, Stelzer said.

In Missouri, a third of its 45 million acres of land is forest, mostly oak and hickory trees. Given the amount of the wood chips cut from whole trees that goes to the university 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 of the Water and Light Department of the City of Columbia. 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.

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The stoker is a machine on the solid boiler’s side, and it’s used to pour the fuels into the boiler. Photo taken in the city power plant by Heidi Li.

Every winter and summer, the city power plant burns coal, wood chips and natural gas in its own boilers to supplement the bulk of electricity it buys from outside of 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 four-story high metal boxes, where coal, as well as 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 arises inside the boiler and then arrives at a turbine, which is connected to a generator. When the turbine spins due to the pressure of steam, it turns on the generator to produce electricity.

The boilers at the MU campus work the same way as those at the city power plant except that the MU’s new biomass boiler burns only wood chips.

The MU power plant produces electricity for the entire campus, including the University Hospital and the Truman Veterans Hospital, with its combined heat and power system, said Karlan Seville, communications manager of Campus Facilities at the University of Missouri. The system has six boilers, and four of them are coal-fired boilers, which burn mostly coal combined with wood chips and tire waste, Seville said in an email.

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The generator produces electricity. Photo taken in the city power plant by Heidi Li.

“The biomass boiler we put in a year ago eliminates 25 percent of our coal use,” Seville said. The new biomass boiler now burns only wood chips, she said.

The amount of carbon emissions from the MU campus, including the power plant and other energy production such as wind and solar energy, was reduced by 28 percent from 2008 to 2013, and Seville estimated that most of that amount came from the newly-built biomass boiler. How much carbon emissions exactly have been reduced by the biomass boiler is yet to be seen, because the boiler hasn’t been in operation for more than a fiscal year, she said in an email.

Policy conundrum and other alternatives

Other than the slow regrowth of trees, policy incentive for burning wood for energy may also lead to an increase of carbon emissions.

Traditionally, the amount of bio-power plants, which are power plants that burn only biomass, is affected by the forestry industry of an area, said Thomas Johnson, professor of agricultural and applied Economics at the University of Missouri.

According to a report by Biomass Magazine, Minnesota has 11 bio-power plants, whereas Missouri has none. Because Minnesota has more forestry resources, they tend to have more bio-power plants,” Johnson said. “In addition, there may be more government incentives in Minnesota than in other states.” The government incentives include both financial and regulatory incentives, he said.

If the government incentives don’t take into account the possible harm burning wood can bring to forests, we risk growing the demand for whole trees, Lyutse said.

“You build the demand and create this incentive that will go beyond the potential (sustainable) case,” she said.

There are other biomass alternatives to burning whole trees as well, Lyutse said. “As long as the quantity (of biomass) remains at a sustainable level,” she said.

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Miscanthus pellets are not durable, and they are pulverized before they reach the stoker, Johanningmeier said. Miscanthus is a kind of grass. Photo by Heidi Li.

So far, the university power plant has tested corn cobs and miscanthus in its coal-fired boilers, but it currently only burns wood chips, Seville said.

“If the market opens for other products, we  could consider those too,” Seville said.

The city power plant, on the other hand, had an unsatisfactory experience with miscanthus pellets, which are not durable, Johanningmeier said.

“Miscanthus pellets turned into powder before they reached the stokers,” Johanningmeier said. As concerns were raised about the possibility that burning miscanthus may ignite other fuels in the stoker at the same time, the test was then stopped, Johanningmeier said.

Summer is coming and the boilers in the city power plant are going to operate. As the city power plant is now working with a local company named Enginuity Worldwide, LLC., the boilers might run the company’s test of corn residue fuels this summer, Johanningmeier said.

As a renewable energy, wood can be more easily renewed than coal, while burning it, especially burning whole trees, may still introduce more cabon emissions to the atmosphere. Government incentives that encourage other alternatives than burning whole trees may be the future solution.to a more sustainable environment.

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One thought on “Burning wood for energy leads to debate over its carbon emissions

  1. Pingback: Deforestation in the UK | Climate Etc.

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