A biomass power plant in Portsmouth, New Hampshire. (Photo by PSNH via Creative Commons)
A biomass power plant in Portsmouth, New Hampshire. (Photo by PSNH via Creative Commons)
A biomass power plant in Portsmouth, New Hampshire. (Photo by PSNH via Creative Commons)

After reporting last week on a Midwest biomass group’s proposal to boost wood-fueled heating in the region, reader John Gunn tweeted to tell us “forest biomass GHG emissions are much more complicated than your article indicates.”

He’s right, so we thought we’d take a closer look at the topic of biomass and carbon emissions.

Gunn is a Minnesota native who now heads a Maine nonprofit research lab, Natural Assets Laboratory, that studies forest carbon issues.

“Based on what we’ve found, it’s not a one-size-fits-all solution in terms of emissions,” says Gunn.

Carbon per kilowatt

It’s a misconception that wood fuels are carbon neutral, minus the energy spent to harvest, process, and transport them, he says. When we burn wood, we’re releasing carbon into the atmosphere that might have otherwise been stored in that form for years, decades or centuries.

Gunn co-authored a 2010 study that concluded the amount of carbon released per unit of energy is actually greater for forest biomass than it is for fossil fuels. That’s because wood isn’t a very energy-dense material, which means you have to burn a lot more tons of it to match the energy output of gas or coal.

It’s a controversial claim, one that’s been disputed by wood-fuel advocates. This alleged “carbon debt” can range anywhere from 2 percent to 66 percent, depending on the type of material burned, what it’s displacing, and whether it’s used to generate heat or electricity, Gunn’s paper asserts.

In the short term, burning wood for energy results in a net increase in carbon emissions, he says. Sustainable forestry practices can help well-designed biomass systems repay that carbon debt and then some, but the carbon benefits typically accrue years or decades into the future, he says.

With atmospheric carbon pushing 400 parts per million, policymakers need to be aware of the short-term costs of burning biomass, says Gunn. As a result of the 2010 study, Massachusetts amended its renewable portfolio standard to exclude biomass projects with long carbon payback periods.

Carbon footprint factors

Gunn is co-author of a more recent paper that identified four factors that were most important in calculating the “debt-then-dividend” curve for a region or biomass facility.

Feedstock: What are you burning, and what would have happened to it otherwise? “You need to look at what was the fate of that material,” says Gunn.

Increased demand for wood fuels might motivate a logging company to harvest a few more trees than it would have otherwise. Under business-as-usual those trees would have continued to store carbon in the forest, but instead the carbon would be released into the atmosphere.

Whole trees produce a greater carbon debt than only using the tops and limbs of trees harvested for other purposes, but even those forest leftovers affect the carbon equation. Left on the forest floor, those branches might decay over several years, releasing some carbon into the air and depositing some carbon back into the soil.

On the other hand, intercepting scrap wood that was bound for a landfill can contribute to a quick carbon benefit, but Gunn said that also depends on a lot of factors.

Heat vs. electricity: The life-cycle carbon emissions from generating electricity at a utility-scale biomass facility are about three times greater per MWh than emissions from a similar-sized natural gas electric power plant and 50 percent greater than a coal-fired electricity plant, according to Gunn’s research.

On a carbon basis, wood pellets compete with coal and natural gas much better in thermal or combined-heat-and-power facilities. A biomass cogeneration plant would emit just slightly more carbon than a coal cogeneration plant, with the difference being less than 3 percent.

What it’s displacing: Biomass has a hard time competing with natural gas financially, and the same is true for carbon emissions.

“[W]here biomass replaces a relatively GHG efficient fossil fuel like natural gas, the time needed to pay back carbon debts and realize the benefits of biomass can increase substantially,” the paper says.

Forest management: The key to paying off biomass’ short-term carbon debt is having sustainable forestry practices in place that ensure that more carbon is being “re-sequestered” in forests than is being removed for fuel. And those practices need to be in place for the long haul, Gunn stressed.

For example, if private land that is sustainably harvested for biomass today were to be sold in the future to a company that didn’t follow the same practices, it could erase the carbon gains. “If that doesn’t hold, then the whole benefit doesn’t hold,” says Gunn.

Forest owners’ decisions about the intensity and frequency of harvests can either slow or accelerate forest growth, therefore affecting the rate that carbon is recaptured by the forest.

Complex equation

Others have looked at similar variables and reached different conclusions about biomass’ carbon profile.

Dovetail Partners, a nonprofit research group based in Minneapolis, for example, concluded in a 2012 study that biomass harvesting gives landowners an incentive to maintain forests rather than converting them to agriculture.

Heating the Midwest, in its report calling for a 10 percent thermal biomass goal by 2025, promoted wood fuels’ potential to reduce global warming, but also acknowledged the complexity.

“The degree to which biomass energy system can reduce carbon emissions compared to fossil fuels is directly related to establishment and management of harvesting regimes, forest types, fuel transport, and efficiency,” the biomass group’s paper says.

It may be on the right track by focusing on sustainably harvested wood for thermal energy in areas not currently served by natural gas, but even so, Gunn says it’s not a given that its vision would be beneficial or even benign in terms of greenhouse gas emissions.

Gunn says he’s not out to attack biomass — he uses wood fuel to heat his own home — but he thinks its also hazardous for policymakers to assume that biomass is inherently carbon neutral.

“A lot of the studies are finding that there is often a benefit to the atmosphere,” says Gunn, “but it often takes a while to get there.”

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