Solar farms are sprouting up across Wisconsin and the state’s wind resources are slowly growing as Gov. Tony Evers has committed the state to reach 100% renewable electricity generation by 2050. Wind- and solar-related policies are regularly hashed out in often-bitter proceedings before the public utility commission and the legislature.
Yet relatively little attention is paid to the potential and risks of Wisconsin’s arguably largest renewable energy source: biomass.
When heating is considered along with electricity generation, the great majority of Wisconsin’s renewable energy comes from wood and biomass — including the residue from forestry and crops that fuels many industrial boilers, homes and one utility-scale power plant.
Wisconsin is especially well-suited to biomass, since its robust timber, paper, dairy and farming sectors produce waste that can be used to generate heat and electricity, creating economic benefits and efficiencies. A state report noted that as of 2017, wood and biomass produced about 10 times as much of the state’s electric, fuel and thermal energy as hydro, the next largest renewable source other than ethanol.
But biomass is also taking a toll on public health, as a recent study by the Harvard School of Public Health found. In Wisconsin, Minnesota, Michigan and swaths of the eastern and western U.S., the Harvard researchers found wood and biomass were responsible for the largest health burden created by particulate matter from stationary sources — more than from coal or gas. “Biomass and wood were responsible for the fastest-growing share of early deaths” nationwide from stationary fuel sources, the study found.
Biomass and wood are highlighted as climate solutions in the recommendations issued by the Gov. Tony Evers’ task force on climate change in December 2020. A “tier 1” recommendation calls for incentivizing the use of woody biomass for industrial processes, combined heat and power, and district heating, including at hospitals and schools.
The recommendation also suggests investment in Wisconsin’s forestry industry, since wood products store carbon long-term, while the harvesting of wood for timber also generates wood waste that can be used for energy.
“Biomass is a broad category,” explained Tim Baye, professor of business development and state energy specialist at the University of Wisconsin-Madison, who has specialized in biomass since 1984. “It runs the spectrum from diverted organics from a municipality [including sewage and yard waste] to crop residuals to forestry residuals to whole trees. The reality of it is biomass-based power still may globally represent the largest share of any renewable power source. It’s larger than wind and larger than solar. Historically it’s been used because it’s the convenient fuel.”
Low-profile yet prolific
Across Wisconsin, homes, businesses — like the Meister cheese company — and heavy industries often burn biomass in boilers for heat, sometimes generating electricity on-site too. The 50-megawatt Rothschild biomass plant supplies electricity to We Energies customers in partnership with a Domtar Paper Company mill that provides the woody fuel.
According to federal data, a fifth of Wisconsin’s renewable electricity generation came from biomass in 2020, and about 1 in 33 Wisconsin households heat with wood.
While power plants are subject to the Clean Air Act and other federal regulations, the government has less power over industrial boilers and very little oversight of wood-fired residential boilers. The Rothschild plant is the only biomass facility in Wisconsin large enough to trigger federal requirements that it use best available technology to control emissions, said Kristin Hart, the Wisconsin Department of Natural Resources’ chief of air permit and stationary source modeling in the Air Management Program.
“A lot of our wood products facilities in the northern part of the state have a wood-fired boiler as part of their operations, to provide steam for curing the wood, and they use wood waste from their operations to fire those boilers,” Hart said. “Those are smaller, they usually don’t trigger best available control technology. They just have to meet general standards that cover all fuel-fired equipment. Those are mostly promulgated in the ‘70s, and are usually not the most stringent.”
Often environmental and citizens groups pay scant attention to biomass and wood energy. A number of such organizations in Wisconsin, Minnesota, Indiana and Illinois said they focus little on the health or other impacts of biomass and wood energy. That’s likely because biomass is used for heat and hot water more than electricity, and typically in smaller behind-the-meter systems that aren’t involved in utility rate cases or state energy policy.
Biomass is considered a low-carbon fuel source since much of the carbon released when such organic material is burned to produce heat or electricity would have made its way into the atmosphere regardless, and the biomass energy replaces fossil fuels. When the lifecycle of biomass is considered — including carbon emissions involved in collection and transport, the type of organic material and whether plants were grown specifically to produce energy — biomass may not be carbon-neutral or even low-carbon.
Collin Buntrock, forest products team leader for the Wisconsin Department of Natural Resources, described the use of woody biomass for energy as essential to a sustainable forestry industry. Managing healthy forests — and preventing wildfires — entails constant thinning and clearing of trees and debris. Harvesting and processing timber also results in large amounts of discarded wood.
“We have to figure out what to do with all that wood,” Buntrock said. “Wood certainly is a low-carbon fuel, and a remarkable story of renewability. It’s supporting local jobs in rural areas where forestry is the main industry.”
The DNR works with local companies to help them use wood and biomass energy, and also encourages schools and other entities to consider wood-energy systems, Buntrock said.
Industrial operations that process wood — like sawmills, paper mills and wood pellet mills — are likely to use woody biomass in boilers on-site for heat and sometimes behind-the-meter electricity, including through combined heat and power systems. For example, wood waste can be burned instead of gas or coal to create the copious heat needed to dry lumber, Buntrock noted. Logging companies might collect wood they can’t process — including the tops of trees — and sell it on a contract to another facility that can use it for heat.
“The interconnectedness to local forests and economies is integral for wood manufacturing and makes sense from a cost standpoint,” Buntrock said.
The Harvard study looked at the health impacts of PM2.5 — dust fine enough to infiltrate organs — and sulfur dioxide, nitrogen oxide and volatile organic compounds, which are precursors that form and interact with PM2.5.
In 2008 burning biomass and wood accounted for around 14-17% of premature deaths nationwide attributed to stationary sources of pollution, the Harvard study found. But by 2017, biomass and wood increased to 39-47% of those early deaths. The share of early deaths attributed to gas also rose, from approximately 11-14% in 2008 to 21% in 2017.
The proportionate health burden of biomass has increased as coal-fired power plants shut down, and as some industries have switched their boilers from fossil fuels to biomass, the Harvard study found. Buntrock noted that installments of new biomass boilers usually correlate with price spikes for propane, natural gas, coal and oil that would otherwise be used as fuel.
“Clearly the reduction in coal combustion as a source of electricity production has been a good thing for our air quality and public health,” said Clean Wisconsin staff scientist Paul Mathewson. “Now an arguably harder task is working on cleaning up emissions from more numerous, smaller, more dispersed sources [like boilers] as these cumulative emissions become the primary problem.”
Mathewson doesn’t oppose the increased use of biomass, but wants to be sure it’s done in a careful way.
“Decision-making really needs to also consider the public health impacts of biomass as an energy source to ensure that the energy system is not only climate-friendly but is also healthy,” he said. “Once you’ve incentivized something, that energy system will be locked in for a while. Do we want to lock in something that will have pretty substantial adverse health impacts?”
Clean energy advocates have pressed for electrification of building heating, so that when electricity is generated by wind and solar, emissions are eliminated. But electrification and renewables can’t necessarily produce the kind of intense and consistent heat needed for the industrial boilers at many facilities in Wisconsin.
“The study points toward using zero emission renewable energy when possible,” said study author Jonathan Buonocore, a research scientist at the Center for Climate, Health, and the Global Environment at the Harvard School of Public Health. “Some really heavy industry operations — things like kilns, metallurgy — are pretty well recognized as among the hardest sectors to decarbonize.”
Outdoor wood boilers heating many homes in Wisconsin face virtually no regulatory mandates even as they release particulate matter and other toxic emissions, especially when treated wood, plastic or trash are burned.
On its website, the Wisconsin DNR explains that outdoor wood boilers have become increasingly popular ways to heat homes in rural areas as propane, oil and gas prices have risen or fluctuated wildly. The DNR cites a 2006 study that showed Wisconsin having the second-highest number of outdoor wood boiler sales since 1990.
Such residential boilers are not regulated by the state. Hart said local governments or the DNR might demand changes from owners of individual boilers that are the source of many complaints from neighbors, but such cases are rare.
The DNR’s website notes that the outdoor residential boilers — “that look like small utility buildings with a smokestack” — burn more slowly, inefficiently and at lower temperatures than typical indoor wood stoves or indoor boilers, creating more toxic emissions in the process. Since 2015, the U.S. EPA has mandated pollution limits for new outdoor boilers, but units purchased earlier are allowed to keep operating.
Residential boilers are “so varied,” Hart added. “From pellet burners in homes that are super clean, to these outdoor wood boilers that are designed to load them up with lots of wood and let them smolder. That design is terrible.”
There are some state and federal incentives that can help people upgrade their boilers, but it isn’t simple.
“Especially in Wisconsin, in a lot of lower-income rural areas wood is an affordable way of staying warm,” said Mathewson, who grew up with a wood-burning stove for heat in New Hampshire and never thought about its emissions impacts. “It’s not easy to just say ‘switch to something else,’ people may not be able to afford to do so.”
Innovation and opportunity
While industrial boilers typically burn biomass to create heat, biomass like manure and food waste can also be processed in biodigesters that turn it into biogas that is chemically identical to natural gas and can be used for heating and generation, releasing less particulate matter than biomass burning.
There are about 200 such biodigesters in the state, according to the Energy Information Administration. Using biogas emits carbon, but greenhouse gases would have been released in other ways if the material did not go through a biodigester, and the biogas can replace natural gas.
Many Wisconsin wastewater treatment plants, landfills, dairies and other agricultural operations use biodigesters, allowing them to produce gas for heating and sometimes electricity generation, according to a recent study by the University of Wisconsin-Stevens Point.
But economics and lack of technical support can be a significant barrier, noted study author Shiba Kar, an associate professor of human dimensions of natural resource management. Biodigesters on average cost $3 million to install, the study found, and most received no grants.
Kar and Baye note that biogas production could be more feasible if policy mandated that utilities allow local producers to transport biogas through their pipelines for sale.
Similarly, net metering policies that provide more compensation for electricity sent back to the grid could encourage more biogas- and biomass-driven electricity generation by local industries and farms.
Emerging technology can greatly reduce the emissions and improve the economics of tapping biomass — through burning or gasification — for energy, Baye and others said. Baye noted that a process called pyrolysis involves burning at high heat and low oxygen, resulting in very low emissions and producing synthetic gas and oil that can replace fossil fuels as well as creating biochar, a valuable soil additive.
But “most of these technologies are still early stages in terms of commercialization,” Baye said, and scaling up could be difficult without incentives and support.
Kar works with colleges and universities to integrate biomass into their energy plan as well as their curriculum. He noted that many institutions have “school forests” used to teach ecology and forest management, and they could also use the forest for energy.
“It’s raising awareness about this fuel,” Kar said. “We’ve been using [forms of biomass] for a long time, but it’s becoming much more efficient and polluting less and less.”