Closing the biomass carbon loophole

Worldwide, forest fires and forest harvesting are recognized as major sources of the carbon dioxide (CO2) that is warming the climate, causing drought, and melting ice caps. Yet, paradoxically, most state, regional, and federal efforts to reduce emissions treat biomass power plants that burn wood for fuel as “carbon neutral,” with zero net emissions. Massachusetts is now leading the way to correct this mistake, with first-in-the-nation regulations limiting the eligibility of biomass power for renewable energy subsidies based on cumulative CO2 emissions.

The new rules eliminate subsidies for large, wood-fueled power plants that produce electricity only, while rewarding high-efficiency combined heat-and-power facilities, which utilize some of their steam for heat and have lower CO2 emissions per unit of useful energy.

 It never really made sense that biomass power should be subsidized in Massachusetts alongside no-emissions technologies such as wind and solar power, particularly given the goals of the 2008 Global Warming Solutions Act, which mandates a 25 percent reduction in greenhouse gas emissions below 1990 levels by 2020 and an 80 percent reduction by 2050.

Per unit of energy generated, a biomass power plant emits 40 to 50 percent more CO2 than a coal plant, and two to three times more CO2 than a gas plant. This is due to the low energy-to-carbon ratio of wood and its high moisture content, which reduces the efficiency of biomass plants to around 24 percent, compared to the average efficiency of the US coal fleet (33 percent) and that of a new combined cycle gas plant (45 percent).

The biomass industry has justified the treatment of biomass as carbon neutral by claiming to use only waste materials, such as lumber mill shavings, paper mill waste, and “forestry residues”‚Äîthe tops and limbs left over after saw-timber harvesting. Burning such materials for energy, it is argued, emits no more CO2 than does letting these materials decompose, and can therefore be considered having zero net emissions. 

There are two main problems with this argument. First, the decomposition of forest wood emits CO2 over years or decades while combustion emits carbon instantaneously. Second, many new plants are not burning true “wastes,” but are instead accelerating forest cutting for fuel. Particularly in the Southeast and the Northeast, exploding demand for biomass is outpacing the availability of waste wood. The air permit for a 70-mega¬¨watt plant being built in Berlin, New Hampshire, shows the scale of impacts, stating that the facility will burn 113 tons of whole logs per hour, the equivalent to the state’s entire 2006 timber harvest. There are more than 170 recently built and proposed biomass energy plants in the United States, and a massive wood pellet industry is also emerging, primarily to meet European demand for “carbon neutral” fuels to co-fire with coal. A single export-oriented pellet plant harvests more than a million tons of trees a year.

Massachusetts has been at the forefront of the bioenergy boom, with utility-scale biomass plants proposed in Russell, Greenfield, and Springfield. The facilities would burn about 1.3 million tons of forest wood annually, more than the state’s annual commercial timber harvest (the Springfield plant as then proposed would also have burned 255,000 tons per year of construction and demolition wood sorted to remove the most obviously contaminated material). Representing 135 megawatts of new electrical capacity, the three plants would add about 1 percent to the state’s 2008 power generating capacity, but the 1.7 million tons of CO2 they would emit each year would increase power sector emissions by 6.9 percent.

The plants would also be some of the biggest conventional polluters in western Massachusetts, each emitting as much particulate matter and nitrogen oxides as a coal plant of the same size. Further, they would require vast amounts of water for cooling—the Russell plant alone would consume up to 885,000 gallons of water a day from the Westfield River, blowing most off as steam and returning the warmed dregs to the river.

After a cross-section of scientists, engineers, and medical professionals joined local residents in raising concerns about the facilities, the Massachusetts Department of Energy Resources took action. The agency’s own back-of-the-envelope calculations on CO2 emissions and forest cutting had concluded that biomass energy might not be compatible with the emissions reduction mandates of the Global Warming Solutions Act. In November 2009, then-Secretary of Energy and Environmental Affairs Ian Bowles commissioned the Manomet Center for Conservation Sciences to study the forest cutting and greenhouse gas impacts of large-scale biomass plants in Massachusetts.

Even as the Manomet team worked, a broad coalition of activists mounted a ballot initiative campaign that would limit renewable energy subsidies to only those technologies that emitted less than 250 pounds of CO2 per mega¬watt hour, a standard that typical biomass plants, with emissions around 3,000 pounds per megawatt hour, could never meet. While the question was not put on the ballot, the effort significantly increased awareness among voters and politicians about biomass energy and its impacts.

The state-commissioned Manomet study, released in June 2010, confirmed that the approximately 100,000 tons of residues generated by forest harvesting in Massachusetts each year would not be enough to fuel even one utility-scale biomass plant. The study went on to examine the net greenhouse gas impact of using both forestry residues and whole trees cut specifically for fuel, using a forest growth model to determine how long it would take for forests to grow back and soak up an equivalent amount of CO2 as had been released by biomass harvesting and burning.

The Manomet model acknowledged that forests are currently growing, and currently sequestering CO2, and that increased harvesting for biomass fuel degrades forest carbon uptake for decades. The study found that a biomass plant could operate for 40 years, all the while allowing forests cut for fuel to regrow undisturbed, but cumulative CO2 emissions would still exceed those from a same-sized coal plant operated over the same period during which forests had not been harvested for fuel. It would take more than 90 years for forest regrowth to draw CO2 emissions from a biomass plant down to the level of a gas plant. However, the study also concluded that the carbon debt created by small thermal-only and combined-heat-and-power biomass projects could be paid off within a decade or two, due to the greater efficiency of utilizing steam for heat than for electricity generation.

It was clear that by subsidizing biomass electricity, the state had been promoting a technology that actually accelerates transfer of forest carbon into the atmosphere over the very period that climate scientists say we should reduce emissions. The state responded by drafting new rules that would ensure renewable energy subsidies went only to the biopower facilities with the lowest net CO2 emissions. 

Released in August, the final regulations are designed to limit greenhouse gas emissions from biomass energy and protect forests from intensive fuel harvesting. Whether located in or out of state, a biomass power generator that wants renewable energy subsidies from Massachusetts must perform carbon accounting to demonstrate that the cumulative CO2 emissions of the facility over a 20-year period are no more than half those of a natural gas plant. The regulations also establish efficiency standards that can only be met by combined-heat-and-power facilities with steam customers that utilize a large proportion of the facility’s steam for heat.

A carefully crafted carbon accounting framework underlies the regulations. It assumes that forest residue decomposes relatively quickly in nature, and discounts CO2 emissions from this fuel source accordingly. However, the framework weights emissions more heavily when fuel comes from whole trees that would have otherwise continued taking up CO2. To protect forest soils from erosion and nutrient loss, the regulations limit the amount of residues that can be removed from logging sites. 

Massachusetts’ new biomass energy regulations effectively remove utility-scale bioenergy from the state’s renewable energy subsidy program. The rules will affect development of out-of-state plants that want Massachusetts subsidies, and may also influence policy at the federal level. The US Environmental Protection Agency recently convened a panel to advise the agency on the best way to count bioenergy CO2 under the Clean Air Act. The panel converged on a carbon accounting framework that is essentially the same as outlined by the Manomet study and reflected in the Massachusetts rules. The question remains, however, whether the EPA will be willing to stand up to biomass industry pressure and acknowledge that utility-scale biomass energy is not a climate-friendly solution. 


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