Emissions reductions from bioenergy have many variables — study
Source: Amanda Peterka, E&E reporter • Posted: Thursday, February 13, 2014
Bioenergy produced from agricultural residues and energy crops such as grasses yields at least 50 percent carbon savings in a 10-year payback period, while bioenergy produced from whole trees offers no greenhouse gas reductions over 30 years, the study found.
“Ultimately our aim is to give an indication of the types of pathways likely to have systematically smaller or larger climate impacts,” ICCT senior researcher Anil Baral and fuels program leader Chris Malins wrote.
ICCT, a nonprofit organization that generates research with the goal of informing policy, used life-cycle analyses to assess greenhouse gas reductions. The study is innovative in its use of a calculation known as global warming potential to take into account how the various bioenergy methods play out over a 30-year time frame.
ClimateWorks Foundation, an organization that pushes for policies to address climate change, funded the research, and Argonne National Laboratory and European energy company Vattenfall reviewed its results.
The study examined five categories of biomass pathways: short-rotation forestry, forest residues, agricultural residues, forest management practices and dedicated energy crops.
Biomass raw material collected from those pathways can be used either to produce electricity and heat or to produce biofuels. The study analyzed both power and biofuels production processes.
Agricultural residues — the parts of crops left on the ground after harvest — and dedicated energy crops like switchgrass and miscanthus had the greatest greenhouse gas reduction potential.
“This study identifies agricultural residue and dedicated energy crops as feedstocks that can potentially deliver large GHG savings within short time frames and can contribute to climate change mitigation, provided that adequate environmental safeguards are in place,” the study found. “Shorter payback periods mean shorter delays in achieving GHG savings.”
After bioenergy from agricultural residues and energy crops, the study found that bioenergy from forest residues offers some greenhouse gas savings with payback periods of up to 25 years. But producing bioenergy from whole trees through forest thinning, reduced-impact logging and short-rotation forestry yielded no greenhouse gas savings.
The study is the first bioenergy carbon study to include reduced-impact logging in its analysis. An emerging practice in Brazil, reduced-impact logging involves selectively removing a few trees from a natural forest stand for timber production with the goal of minimizing losses to biodiversity.
The study is also unique in its use of global warming potential, a figure that compares the warming impact from temporary biomass emissions to the warming impact over 100 years of emitting the same amount of carbon from fossil fuels production. The GWP factor is higher the longer bioenergy carbon emissions remain in the atmosphere.
The calculation is significant because some inputs, such as grasses, grow back quickly after being harvested, while trees can take tens or hundreds of years to regrow, the authors said.
“Imagine that a 500-year-old tree is felled and burned for bioenergy,” the authors said. “It will take hundreds of years for a replacement tree to grow to the same size, and in the interim period the effect would have been to increase atmospheric carbon. In that interim period, this could be worse for the climate than if coal had been burned instead.”
The authors acknowledge a few limitations. The study does not, for example, take into account all possible types of biomass production methods, nor does it fully account for the complexities that go into managing forests.