Burning forest trees for power can result in an initial “carbon debt” because it releases more carbon dioxide into the atmosphere per unit of energy than oil, coal, or natural gas, before it reduces carbon dioxide, sometimes over decades, according to a six-month-long study commissioned by the Massachusetts Department of Energy Resources (DOER).

The “Biomass Sustainability and Carbon Policy Study” from nonprofit research organization Manomet Center for Conservation Sciences addressed questions such as what the climate science (or “carbon accounting”) implications are of using forest biomass for energy and how biomass harvests could impact forest ecosystems.

It said use of biomass for heating and for combined heat and power (CHP) facilities would result in a 25% reduction in greenhouse gas emissions in 2050 relative to oil, but biomass-fired electricity would result in a 3% increase in emissions over coal-fired electricity in 2050.

In particular, it showed that using wood can result in an initial carbon debt, but unlike fossil fuels, forests can grow back and recapture (or sequester) CO2 from the atmosphere. Over time, through accelerated forest growth, the carbon debt can be “paid off.” After the carbon debt is paid off, if the forest continues to grow, a “carbon dividend” is realized and the use of wood for energy then becomes increasingly beneficial for greenhouse gas mitigation, the organization said.

Carbon Neutrality Is Circumstantial

“As a result, using wood for energy can lead to lower atmospheric greenhouse gas levels than fossil fuels, but only after the point in time when the carbon debt is paid off. Whether or not full carbon neutrality will be achieved in these circumstances will depend on if, when, and how the forest is harvested in the future,” the study noted.

According to Manomet, the length of time it takes to pay off the carbon debt and begin accruing carbon dividends (i.e., greenhouse gas benefits) can vary widely, from five years to many decades. The length of time depends on a complex interaction of the type of biomass energy used (electricity, heat, or combined heat and electricity); the fossil fuel that biomass energy replaces (coal, oil, or natural gas); and the degree to which the growth potential of the forest is realized by the landowners’ forest management methods.

“As an example, with an electric power plant that relies on biomass using whole trees from natural forests in the Massachusetts region—and not waste wood from tree work and landscaping that has different carbon cycle impacts—the carbon debt period is likely to last for at least 20 or 30 years before carbon benefits begin to be realized,” Manomet said in a press release associated with the study. “In contrast, using forest biomass in thermal applications, such as heating municipal buildings or schools, has lower carbon debts and can provide carbon dividends for the atmosphere sooner, generally within 10 to 20 years.”

Context-Specific Results

“Understanding the greenhouse gas impacts of woody biomass energy is extremely complex,” said Thomas Walker, the study team leader. “We’ve come up with a comprehensive but relatively straightforward framework and methodology that can help policy makers in Massachusetts, or anywhere, better understand if and when any particular wood biomass energy scenario might be ‘climate friendly.’” Walker stressed, however, that while forests in other geographic areas can be evaluated similarly, they would yield context-specific results with respect to greenhouse gas costs and benefits.

Nevertheless, the study’s results will likely have broad implications for Massachusetts’s energy landscape. Biomass has qualified for incentives under the Massachusetts Renewable Portfolio Standard (RPS) since the RPS was created in 2002 under the Electricity Restructuring Act of 1997. The RPS is supposed to create financial incentives for eligible renewable energy facilities by requiring utilities to deliver a minimum percentage of RPS-qualified renewable energy to their customers. The Green Communities Act of 2008 accelerated the rate of increase of this percentage (currently 5%) from 0.5% to 1% annually.

In a press release last week, Secretary of Energy and Environmental Affairs Ian Bowles said, “now that we know that electricity from biomass harvested from New England forests is not ‘carbon neutral’ in a timeframe that makes sense given our legal mandate to cut greenhouse gas emissions, we need to re-evaluate our incentives for biomass.”

That statement prompted the Biomass Power Association (BPA) to request an apology and a clarification from Manomet Center for Conservation Sciences and the Commonwealth of Massachusetts for what it called a “flatly misleading, irresponsible, and not an accurate portrayal of the [biomass] industry.” The industry group claimed that the study bases its analyses on new forest biomass instead of the waste wood that most biomass plants use or propose to use. That waste wood would be "dumped in landfills, openly burned, or left as fodder for forest fires," it said.

“The report unfortunately, completely ignores these fuels in the first 109 pages of the report and it’s not until page 110, almost an afterthought, recognizes the fact, ‘All bioenergy technologies, even biomass electric power compared to natural gas electric, look favorable when biomass waste-wood is compared to fossil fuel alternatives,’” said Bob Cleaves, president and CEO of the BPA.

Cleaves pointed out that biomass provides 15% of the nation’s renewable power and that the Department of Energy expects biomass consumption to double every decade through 2030. He said it would be difficult to meet ambitious renewable energy goals without biomass.

Sources: POWERnews, Manomet Center for Conservation Sciences, Massachusetts’s Governor’s Office, BPA