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BURLINGTON, Vt. — When it purchased a small hydropower plant on the nearby Winooski River last fall, the municipally-owned electric utility here quietly nudged Vermont's largest city into the sustainability spotlight. With a little creative accounting and the addition of the Winooski facility, Burlington's 42,000 residents were now lighting their homes and running their businesses with a 100-percent renewable mix of wind, water and biomass.
That last resource raised some eyebrows, however, and not everyone was celebrating. Mary S. Booth, a New England ecologist, fired off a letter to the editors of the “PBS Newshour,” which was among many news outlets to herald Burlington's achievement. She pointed to the city's McNeil Electric Generating Facility, which burns wood to produce about one-third of the city's power, noting that the plant emits a variety of pollutants, including greenhouse gases.
Being renewable, Booth suggested, is not the same as being clean and carbon-free.
In an editor's note attached to the segment online, PBS suggested that Booth was not alone in her objections, and it modified its story to reflect concerns raised by several members of its audience. “These viewers argued we were giving an overstated impression of the environmental attributes of the [McNeil] plant,” PBS noted, “and we agree.”
The backpedaling seemed to underscore a variety of lingering uncertainties over the green credentials of biomass power, and the extent to which Burlington's path away from fossil fuels can — or even should — be emulated elsewhere. The benefits of biomass power depend on such a multitude of variables, in fact, that officials from the U.S. Environmental Protection Agency to the European Commission are still trying to sort out the upsides from the down.
Hanging in the balance is the biomass power industry itself, which is considered climate-friendly in some circles and a climate menace in others. Operators of the McNeil facility argue that Vermont's wood-harvesting regulations are such that their plant, and others like it, can provide what is essentially carbon-free electricity. In neighboring Massachusetts, however, a multi-year debate over the issue concluded in 2012 with what are widely considered to be the toughest biomass regulations in the country.
The new rules effectively shut down the industry there.
“It's frustrating,” said Bob Cleaves, the president of the Biomass Power Association, a Maine-based trade group representing the industry. “We are getting swept into a much larger debate about forests and carbon stocks and spatial and temporal metrics and the like,” he said, “and what we're trying to say is that it's not that complicated.”
Deriving electricity by burning wood or agricultural residues to heat water, create steam, and drive a turbine, was long considered to be both carbon-neutral and a sustainable source of energy. That's because while trees and plants do release large amounts of planet-warming carbon dioxide when they are burned — even more than coal, by some measures — they also absorb and trap the gas while they are growing. So long as the harvested biomass is replaced by new plant or tree growth, the thinking goes, no net emissions are added to the atmosphere.
But the presumed benefits of biomass power — particularly when derived from woody materials taken from forests — have come under increasing scientific scrutiny, and conservation groups are now calling for tighter controls to ensure that policymakers are getting the carbon balance right.
Much of the concern focuses on the time frames needed for new tree growth to soak up carbon dioxide and balance that atmospheric ledger — a question that can hinge on a variety of site-specific conditions, including the local climate, the type of material being used for energy, and what would have happened to that material if a biomass plant hadn't created a demand for it in the first place. There is also little agreement on how long is reasonable to wait for the carbon benefits of biomass to be realized — particularly given the imperatives of global warming. Is 50 years too long to wait? What about a century?
Most experts agree, for example, that generating electricity by burning sawdust and other waste materials collected from the wood-processing and paper-making industries has a low-carbon footprint. Left alone or placed in a landfill, these waste materials would decompose and release their stored carbon back into the atmosphere relatively quickly anyway, so burning them to generate electricity — or even heat — creates little in the way of additional greenhouse gas emissions.
The same is generally true for urban scrap wood, woody debris left behind by storms, trimmings from power line maintenance, and the tangle of tree tops, branches and other “slash” left over when forests are logged by the paper and timber industries — so long as a certain amount of that debris is left in place to provide important nutrients to the soil and to limit erosion.
But when biomass fuels move beyond these waste streams and target whole trees, things get complicated. Harvesting and burning trees specifically for energy production, for example, can throw the carbon benefits off completely. In such cases, even if new trees are replanted, it can take many decades — and even a century or more in some cases — for the new growth to cancel out the emissions from what was harvested and burned.
That sort of time frame strikes many stakeholders as dangerously long given the imperatives of climate change.
A grayer area involves trees that are taken down as part of ordinary forest maintenance — thinnings and treatments aimed at addressing insect infestations, for example, or limiting fire risk. Some studies suggest these might have near-term carbon benefits, though much depends on the severity of the infestation, the size of the fire risk and other factors.
And all of these caveats depend on a nearly endless and ever-shifting assortment of related contingencies: The overall growth rate of a forest tract, for example, the local climate where the wood is harvested, changes in soil composition, and even the type of electricity being displaced. If biomass is substituting for coal, the benefits from the switch might be realized more quickly, for example, than if the alternative was natural gas.
Also at issue: The efficiency of the biomass plant in question. Only about 25 percent of the energy potential of wood is converted to electricity in a typical biomass plant, for example — less than even a typical coal-fired plant — because so much energy is lost as waste heat. That efficiency rate can be boosted significantly — and the carbon footprint of a plant greatly reduced — by capturing that waste heat and using it to keep homes and businesses warm. But this requires a year-round market for heat, so few plants do it.
A study produced in 2014 by the American Lung Association and the Massachusetts-based Partnership for Policy Integrity, a group co-founded by Booth, analyzed 88 pollution permits for biomass power plants in 25 states. It not only concluded that the plants often emitted more local air pollution than comparable coal-fired plants, but that they also release as much as 50 percent more carbon dioxide.
“Current science shows that while emissions of CO2 from biomass burning can theoretically be offset over time by forest regrowth and other means, such offsets typically take several decades to fully compensate for the CO2 emitted during plant operation," the study authors concluded. “None of the permits analyzed in the report required proof that carbon emissions would be offset.”
A study completed in 2010 by the Massachusetts-based Manomet Center for Conservation came to similar conclusions, noting that biomass power generally comes with an initially large carbon footprint, which is only eradicated over time — and only if future production remains sustainable.
Those findings were ultimately enshrined in new 2012 biomass guidelines in Massachusetts. These established tough requirements for the efficiency rates of new biomass plants; set narrow limits on what sorts of material could be used to produce biomass power; and put in place strict guidelines for, and comprehensive oversight of, the biomass harvesting process — all to ensure that biomass activity didn't set back the state's emissions reduction goals over coming decades.
Environmental groups point to Massachusetts as a model for thoughtful biomass regulation. "Time frames matter," said Sami Yassa, a senior scientist with the Land and Wildlife Program at the Natural Resources Defense Council. "If you clear cut an old-growth forest and burn it, it will take 100 or 200 years to reverse the climate impacts," he said. "This is ridiculous, and policy imperatives should require short-term time frames."
These sorts of considerations are now being weighed by the U.S. Environmental Protection Agency, which is taking input from scientists, industry and conservation groups on precisely how to account for the carbon emissions of biomass and other forms of bioenergy. The agency floated a new draft of its proposed accounting framework in November, and is now receiving feedback from stakeholders.
“We have made no definitive statements on what the role of biomass will be,” EPA spokeswoman Liz Purchia emailed, when asked to elaborate on the agency's proposed framework. “We expect certain waste products and forest-derived waste products might be OK, but that doesn’t mean all forest products.”
Scientists and conservation groups have also pleaded with European policymakers to reconsider current carbon accounting for biomass there. At the moment, all biomass is essentially treated as being carbon neutral, regardless of the fuel source or the circumstances of its harvesting. This has led to an explosion of wood-burning biomass power plants — and more worrying, the import of millions of tons of processed wood pellets from forests in the American Southeast.
“Demand for wood pellets in Europe is fueled by misguided energy policies,” a group of five-dozen scientists, including the prominent American biologist E.O. Wilson, declared in a 2013 letter to European energy officials.
Several industry and environmental stakeholders have been meeting under the auspices of Britain's international and policy research think tank Chatham House, in an effort to shore-up outdated biomass policies “based on the incorrect assumption that its use is immediately and completely carbon-neutral.”
A report on the matter is expected later this year.
All of this deliberation, however, can strike members of the biomass power industry as fruitless speculation. As they see it, so long as the overall volume of living, growing, carbon-absorbing biomass is kept stable — or even increasing — biomass power should present no additional threat to the climate.
"I think it was bullshit," said John Irving, the chief engineer and plant manager at the McNeil plant in Vermont, when asked about the Manomet Center study and its implications. As an avid outdoorsman and ocean sailor, Irving says he's deeply concerned about the impacts of human-driven climate change, and he believes that biomass power has a role to play in addressing the problem. Roughly 70 percent of the wood used at the McNeil plant, Irving noted, comes from non-merchantable trees and harvest residues, with the balance coming from purchases of sawmill residues and urban wood waste — precisely the sorts of sources that scientists suggest can provide real carbon benefits in a relatively short timeframe.
"If you build the plant correctly and harvest appropriately," Irving said, "it's a viable renewable source. "
"There's no perfect renewable," he added.
Environmental groups don't necessarily disagree with Irving's take. They just want to see stricter guidelines put in place — locally, nationally and internationally — to ensure that biomass power producers, forest managers, loggers, landowners and everyone in the woody biomass supply chain, adhere to strict rules. Without these, some stakeholders worry, the biomass industry will inevitably outgrow its waste-wood fuel supply and begin competing for whole trees with the timber and paper industries, launching an expansion of logging that would worsen the climate problem in the long run.
Bob Cleaves of the Biomass Power Association says this is an unfounded concern. "Biomass power in the United States uses residues and byproducts, period," he wrote in a letter published in the Washington Post last month. "There is no market to sell timber for fuel."
The debate is certain to continue.
"It's been interesting to watch the arc of things," said Ethan B. Davis, an energy analyst and biomass consultant for the Cambridge-based Union of Concerned Scientists (UCS). "I've rubbed shoulders with really strong biomass proponents and am convinced that they have their hearts in the right place and believe that bioenergy is really going to solve a lot of problems. And on the other side are a lot of passionate and smart people who feel just the opposite."
Whether some carbon-neutral, environmentally friendly middle ground can be found between the two perspectives, Davis says, is an open question. The science, he suggests, is still being sorted out, and while UCS characterizes biomass as having the potential to help address the climate problem, the devil is very much in the details — and the details will be challenging to get just right.
If the science suggests that separating good biomass from bad "is so site-specific that it's virtually impossible to have any broad policy," Davis said, "then maybe the risks outweigh the rewards."
For her part, Booth has already come to that conclusion.
"I think a forester should be able to cut down a tree if he wants to, and I think he should be able to burn it in a boiler if he wants to," Booth said. "I just don't think it should ever be called carbon neutral."