Effect of biogenic carbon inventory on the life cycle assessment of bioenergy: challenges to the neutrality assumption

Abstract

Biogenic carbon is defined as carbon contained in biomass that is accumulated during plant growth. In spite of the considerable progress towards the inventory of biogenic carbon in the life cycle assessment (LCA) of bioenergy in policy guidelines, many scientific articles tend to give no consideration to biogenic carbon, due to the neutrality assumption, rather than to employ a complete inventory according to the LCA principles. Meanwhile, the assumption of biogenic carbon neutrality has been previously challenged on the basis of changes in soil carbon stock due to land use change and carbon storage capacities of long-rotation trees or wood products. Supporting this argument, we investigate three other inventory aspects which strongly affect final results, namely, differences in framing system boundaries (cradle to grave vs. cradle to gate), forms of carbon emissions (carbon dioxide vs. methane), and valuation of biogenic carbon (relative economic values of biomass products vs. biomass residues). Referring to a generic bioenergy system, our analysis is focused on eight scenarios of various carbon flows encompassing biomass decomposition in fields and its alternative utilization as bioenergy feedstocks. These scenarios are applicable to both biomass products and biomass residues, for which the impacts proportionally depend on the chosen allocation criteria between the two. Further, a framework to quantify the performances of the various possible carbon flows on global warming impacts is formulated. The operation of the framework demonstrates that the assumption of biogenic carbon neutrality introduces a bias to the ‘true’ values based on a complete inventory. This can make the values of global warming impacts substantially higher or lower than the real scores when different system boundaries, forms of carbon emissions, and biomass valuation are taken into account. The results of this study could contribute to the harmonization of future bioenergy LCA by directing further research to adopt more the concept of utilizing a complete inventory rather than the neutrality assumption.