Abstract
Understanding the influence of method choices on results in life-cycle assessments is essential to draw informed conclusions. As the climate impact of bioenergy remains a debated topic, the focus of this study is how the chosen temporal framing influences a comparison of the climate impact of utilizing residual biomass for biofuel production to that of leaving the biomass to decay. In order to compare the biofuel scenario to its corresponding reference scenario where biomass is left to decay, a variety of analytical approaches were used: using time-aggregated and time-dependent life-cycle inventories and climate-impact assessment methods, assuming biogenic carbon to be climate neutral or not, using metrics for cumulative or instantaneous climate impact, and with different time horizons. Two cases of residual biofuel feedstocks were assessed: logging residues from Norway spruce forest, and straw from wheat cultivation. Consideration of the studied method choices appears to be especially relevant for forest residual biomass, as illustrated by the ranges of parity times for logging residues (25 to 95 years), and the results which vary with the chosen climate-impact metric, time-horizon, and approach for including biogenic carbon. Illustrating the time-dependence of results can, in general, provide a better understanding of the climate impact of utilizing residual biomass for biofuels.
Highlights
The body of literature on the application of life-cycle assessments (LCAs) to bioenergy has grown rapidly over the past couple of decades [1], as has the implementation of the knowledge gained from LCA in policies and industrial applications [2]
Several of the tested approaches have the possibility to change the outcome of climate impact comparison: biogenic CO2, both its inclusion and the method chosen for its inclusion, the choice of time-horizon, the choice of climate-impact metric (AGWP or AGTP), and choices related to system design and the data used
The results show that different conclusions regarding the preferable scenario for logging residues can be drawn depending on how biogenic CO2 is included, the choice of time-horizon, and the choice of a cumulative or instantaneous metric for climate-impact
Summary
The body of literature on the application of life-cycle assessments (LCAs) to bioenergy has grown rapidly over the past couple of decades [1], as has the implementation of the knowledge gained from LCA in policies and industrial applications [2]. The representation of biomass life cycles and fluxes of greenhouse gases (GHG) over time is important to the resulting climate impact of biofuels [6,7,8], and the temporal framing of climate-impact results affect how different emissions with climate impact are weighed together into a single result in characterization [9,10,11] These issues have led to suggestions to account for temporal aspects in LCA [12,13] and to avoid the assumption that biomass is inherently carbon neutral [14,15] by including biogenic carbon dioxide (CO2) and soil organic carbon (SOC) [16]. Several climate-impact metrics and time horizons should be tested in each study in order to reveal the uncertainty arising from the choice of method used [9,17,18,19]
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