Nov 18, 2021 8:00 AM

How 'Green' Are Wood Pellets as a Fuel Source?

Europe is betting big on wood to replace coal, but the industry is taking heat for stoking carbon emissions and air pollution.

In the search for a clean source of energy, a contender has emerged: little pellets of compressed wood. Harvested from forests in the American South, pine and hardwood trees are dried, compressed, and turned into inch-long pellets that are burned as fuel in electric power plants, mainly in the United Kingdom and Europe, to power homes and businesses.

Under rules grandfathered into the Paris Climate Agreement and reaffirmed this summer by European regulators, burning trees for electric power is considered a carbon-neutral energy source—as long as the trees are replanted. The wood pellet industry argues that it provides an alternative to coal and relies on a sustainable resource: forests that will regrow in the future and remove carbon from the atmosphere.

But many scientists and conservation groups say the opposite: that burning wood is as dirty as coal, and the claim of carbon neutrality is an error that will boost emissions and make it impossible to keep the planet from warming further. What’s more, trees that are cut down take anywhere from several decades to half a century to regrow, time that many climate scientists say the planet doesn’t have.

“It's really simple,” says John Sterman, professor of management and director of the sustainability initiative at the MIT Sloan School of Business. “You put carbon into the air right now, today. But regrowth takes time and is not certain. Maybe you'll remove it in decades to come, or a century from now. That's a terrible deal.”

In 2018, Sterman and two colleagues published a study in the journal Environmental Research Letters calculating that carbon dioxide emissions from burning wood are actually higher than burning coal because wood contains more water—even when dried and compressed into a pellet—and is a less efficient source of energy. The study stated that it would take 44 to 104 years for new growths of trees to soak up that excess CO₂and make wood a greener fuel source than coal. (The wide range in the study indicates that some forests regrow faster than others.)

In Washington, provisions to boost the wood pellet industry were included in the massive $1.2 trillion infrastructure bill signed Monday by President Joe Biden, as well as the $1.75 trillion Build Back Better spending bill being negotiated by Congress. On November 4, a group of 100 forest ecologists, climate scientists, and ecosystem experts, including Sterman, signed an open letter to Biden and Congress urging them to remove these provisions in both pieces of legislation.

The infrastructure bill, which is now law, approves an additional 30 million acres of logging on federal public lands over the next 15 years. It also exempts logging for wood pellet plants from the National Environmental Policy Act, which requires the US Forest Service to study the environmental effects of its own proposed actions before it makes a decision. That new law also adds subsidies to promote carbon capture and storage (CCS) technology at plants that both manufacture and burn wood pellets.

As for the bigger reconciliation bill, which is still being negotiated, language also would subsidize logging on both federal and private lands, as well as subsidize forest biomass energy, wood pellet facilities, and production of cross-laminated timber (a type of prefabricated wood panel used in home construction) under the heading of “wood innovation.”

In their letter, the scientists wrote that encouraging more commercial logging and wood-fired electricity “ignores the advice of hundreds of climate and forest scientists who have previously informed Congress that these industries substantially increase emissions and worsen the climate crisis.”

But not all experts agree on that point. Bob Abt has been researching the ecology and economics of southern forests for more than 40 years, and is an emeritus professor of natural resources at North Carolina State University. He says that, under the right economic and environmental conditions, the carbon footprint of wood pellets can be smaller than coal’s. Making this equation work—so that the amount of carbon being burned for electricity today is offset by future tree growth—has a couple of requirements. First, Abt says, owners of timberland have to harvest fast-growing trees, such as the pines or mixed hardwoods found in the South. The same process would not work as well in forests in New England or the Pacific Northwest, which take much longer to regenerate.

The second thing is making sure landowners who sell wood to pellet companies continue to keep their land in production as working forests. Abt says that as the demand for wood-to-energy increases, so will prices for the wood. That will serve as an incentive for timber owners to keep their trees growing until maturity, rather than turning that same land into pastures for livestock grazing or farmland for seasonal crops, or selling it to housing developers. A 2017 study by researchers at the Oak Ridge National Laboratory found that sprawl from housing tracts and shopping malls could also endanger those forests. “Urbanization—currently the greatest cause of forest loss in the Southeastern US—is more likely to expand into forest landscapes if forest landowners lack adequate income generating opportunities for their wood,” the report stated.

If the land the wood pellets are harvested from is later converted to other uses, then any carbon released today by burning the pellets for electricity will not be recaptured by those trees in the future. That means that the wood pellet industry’s claims of carbon neutrality might depend on the price owners in North Carolina, Georgia, or Mississippi can get for their land—something that is difficult to predict decades into the future.

Abt says that using forests for energy might not be perfect, but it’s a step in the right direction. He says all solutions to the climate crisis need to be on the table. “For wood coming out of the south,” he says, “I am comfortable saying that it's better than coal under most circumstances.”

In places like the United Kingdom, which doesn’t have domestic supplies of natural gas, there’s been a big push to burn wood pellets. In fact, UK-based Drax converted the island nation’s biggest coal-fired power plant in North Yorkshire into a pellet-burning plant in 2013. It now produces enough electricity for 4 million homes, with wood pellets imported from the US. Drax currently operates 13 pellet plants in the US and Canada, and is building another three in Arkansas, according to Ali Lewis, head of media and public relations at Drax.

Lewis says companies like Drax use tree tops, underbrush, and smaller branches from each tree, material that is often discarded by other wood-based industries such as lumber or paper production. Thinning out trees and brush helps keep the forest resistant to both insects and fire, she says. “The science around sustainable managed forestry is clear—actively managing the forests provides economic and environmental benefits—it results in better quality trees, more wildlife and healthier forests,” Lewis wrote in an email to WIRED. “It is also vital in the protection of forests against wildfire, pests, and diseases.”

Lewis says Drax plans to install biomass energy with carbon capture and storage (known as BECCS) technology at several new proposed plants in the UK and Europe. The first BECCS unit at Drax could be operational in 2027, with a second running in 2030. The idea is to capture carbon dioxide emissions from the pellets before they escape to the atmosphere, turn the gas into a liquid form of CO₂, and then pipe it to a permanent storage site at the bottom of the North Sea, according to Drax.

Lewis says each new BECCS plant will capture four metric tons of CO₂ per year. “The combined eight metric tons will make Drax’s carbon capture project the largest CCS project in the world,” Lewis wrote. “This will also mean Drax will be capturing more CO₂than is emitted across its entire operations, creating a negative carbon footprint for the company.”

Over the past three years, Drax and Maryland-based Enviva, the world’s largest pellet producer, have drawn fire from environmental groups in the US and UK who say the facilities emit pollution that affects communities of color who live nearby. In February, Drax was fined $2.5 million by Mississippi state regulators for violating emissions limits for volatile organic compounds produced during the processing of wood pellets. The firm accepted the fine and promised to install new air pollution control equipment to fix the problem at the Mississippi plant. "We take our environmental responsibilities seriously and we are committed to complying with all local and federal regulations,” a Drax spokesperson told the BBC. "The safety of our people and the communities in which we operate is our priority.”

Enviva agreed to do the same thing—add anti-pollution devices—to a North Carolina plant after agreeing to a settlement from a 2019 lawsuit filed by the Southern Environmental Law Center, a legal advocacy group based in Charlottesville, Virginia, and two other advocacy groups. The lawsuit alleged that North Carolina environmental officials had failed to properly scrutinize Enviva’s operating permit. In June, state officials ordered the company to reduce volatile organic compound emissions by 95 percent at a second Enviva wood pellet facility in North Carolina, according to a report in The Fayetteville News and Observer. (Enviva representatives did not respond to WIRED’s requests for comment.)

The fast-growing wood pellet industry is problematic for several reasons, according to Heather Hillaker, staff attorney for the center. Plants are burning wood, putting carbon dioxide into the atmosphere today and emitting air pollutants into nearby neighborhoods, she argues. She also says investment in this kind of technology is diverting public money that should be put into renewables like solar, wind, and battery storage.

“Something that is going to increase CO₂ emissions in the short term, like biomass, it's just not a solution to climate change,” Hillaker says. “We need to be looking at solutions that are going to be reducing CO₂ emissions immediately, while also increasing our ability to store carbon.”