The USDA-NIFA, through NARA, funds research to determine the environmental impact from a wood-based biofuel and chemical industry. To gauge how this industry would affect greenhouse gas (GHG) levels, a life cycle assessment (LCA) is being developed. In this case, the net amount of air emissions generated from harvesting forest residuals through to product development, a span often referred to as “cradle to grave”, will be determined. The results are used to compare the global warming impacts of one conversion process to another. For instance, NARA will compare the amount of greenhouse gas (GHG) emissions produced during the jet fuel production process from using forest residuals relative to petroleum. These comparisons weigh heavily in global strategies to reduce the level of GHG emitted into the atmosphere.
Calculating the net emission from an industrial process using woody biomass is complicated since CO2 is being sequestered and released in the forest over a long time period. To address the temporal flow of CO2 in a working forest, NARA researchers at the University of Washington and the University of Padua in Italy recently published an analysis that incorporates the temporal aspects of carbon sequestration within the LCA framework.
View Evaluation of environmental impacts of harvest residue-based bioenergy using radiative forcing analysis here.
The composition of working forests and how they are managed vary, so the authors based their initial evaluation on a typical Pacific Northwest harvesting operation in Grays Harbor County, WA using a rotation age of 45 years. A whole tree harvesting method was employed where the felled tree is hauled to a landing before the limbs and tops are removed. The study assumes that 61% of the above ground biomass harvested is used for timber products (logs) and 39% is residual biomass (primarily, tops and branches). Further, the study bases its calculations on the assumption that one bone-dried metric ton (BDmT) of residual biomass at the landing site is derived from 1.7 BDmT of total residual biomass harvested. It was assumed that 0.7 (35%) of the 1.7 BDmT harvested tree remains on the forest floor or at the landing due to limbs breakage during harvest and hauling.
Calculating net global warming
To be able to maintain the focus on biomass-based bioenergy, all the carbon balance calculations in the study are based on the residual biomass and the carbon credits or debits associated with timber products (e.g., lumber) are excluded using mass allocations principles. The authors calculated the GHG outputs generated from the harvesting, processing and transporting of the forest residuals to a bioenergy refinery. They also calculated the rate of decay of the residuals left on the forest floor. It was assumed that over time 90% of the residuals decomposed and generated carbon emissions into the atmosphere while 10% decays into the soil. In addition, the emissions generated from burning the residuals to produce energy were determined.
These emission values were used to calculate the accumulated abundance of GHG (as CO2 equivalents) over a 100-year time frame, which is the recommended time-frame set by the Intergovernmental Panel on Climate Change (IPCC) for the assessment of the impact on climate change. Over this time period, the working forest sequesters (takes up) CO2, and these values are also incorporated into the assessment. When CO2 emissions and carbon sequestration are plotted on the same graph for a 100-year time frame, a “Radiative Forcing Turning Point” is obtained at year 18. At this point, the cumulative negative radiative forcing from CO2 sequestration (i.e., global cooling) offsets the cumulative positive radiative forcing from CO2 emissions (i.e., global warming). In other words, the system goes from being “carbon positive” and emitting carbon into the environment to “carbon negative” and storing carbon from the atmosphere. Addressing this result, the authors state, “This study shows that carbon sequestration plays a significant role in the carbon balance and demonstrates the environmental benefits of using woody biomass-based bioenergy.”
International standards allow LCAs to assume carbon neutrality, for global warming indicator calculations, as the CO2 generated from burning wood-based products is offset by CO2 sequestration within the forest. The concept of “carbon neutrality” contributes significantly to the favorable greenhouse warming potential calculated for wood-based biofuels compared to fossil-based fuels. This study introduces a temporal component to the carbon neutrality aspect of LCA and demonstrates the significance of forest management practices on net GHG emissions.
The authors acknowledge that this study does not account for the role of below-ground biomass and natural disturbances like fire in affecting the carbon balance. In addition, this study is based on a limited geographic area and not reflective of a larger more diverse region.