New studies raise doubts about greenness of biomass

Although carbon math is complex, critics level two main strikes against biomass. First, because of its high moisture, wood yields less energy compared with more efficient fuels. Generating the same amount of electricity from biomass emits 45 percent more carbon dioxide than coal and almost 300 percent more than an efficient natural-gas power plant, according to a 2010 study by the Manomet Center for Conservation Sciences, a nonprofit environmental research organization. The study was commissioned by the Massachusetts Department of Energy Resources. Second, burning biomass releases carbon dioxide instantly, while repaying that carbon debt through new tree growth takes years. Just how long depends on many factors, including the biomass source, what was done with it before and the fossil fuel it displaces. Replacing coal-fired electricity by burning tree tops and other wood waste, for instance, might take 10 years to recoup the carbon, said Thomas Walker, a resource economist and team leader of the Manomet study. But if whole trees were harvested for feedstock — something the industry says it doesn't do — and the resulting electricity replaced cleaner-burning natural gas, Walker said, payback might take a century. The timing matters because the nation has pledged to reduce greenhouse-gas emissions by 17 percent from 2005 levels by 2020 and dramatically more in subsequent decades...more