Abstract
Carbon dioxide (CO2) emissions from biomass combustion are traditionally assumed climate neutral if the bioenergy system is carbon (C) flux neutral, i.e. the CO2 released from biofuel combustion approximately equals the amount of CO2 sequestered in biomass. This convention, widely adopted in life cycle assessment (LCA) studies of bioenergy systems, underestimates the climate impact of bioenergy. Besides CO2 emissions from permanent C losses, CO2 emissions from C flux neutral systems (that is from temporary C losses) also contribute to climate change: before being captured by biomass regrowth, CO2 molecules spend time in the atmosphere and contribute to global warming. In this paper, a method to estimate the climate impact of CO2 emissions from biomass combustion is proposed. Our method uses CO2 impulse response functions (IRF) from C cycle models in the elaboration of atmospheric decay functions for biomass-derived CO2 emissions. Their contributions to global warming are then quantified with a unit-based index, the GWPbio. Since this index is expressed as a function of the rotation period of the biomass, our results can be applied to CO2 emissions from combustion of all the different biomass species, from annual row crops to slower growing boreal forest.
Highlights
BackgroundIn 1991, the first comprehensive guidelines for estimating national greenhouse gas (GHG) emissions and sinks compiled by the Organization for Economic Cooperation and Development (OECD) states that ‘CO2 emissions resulting from bioenergy consumption should not be included in a country’s official emission inventory’ (OECD, 1991)
The effect of the rotation length on the full IRF (FIRF)-based decay is shown in Fig. 4, where the bio CO2 fraction remaining in the air after a pulse emission is reported as a function of time and biomass rotation period
The work performed in this paper brings a new contribution to the rising discussion on the proper accounting of CO2 emissions from biomass combustion in bioenergy systems
Summary
In 1991, the first comprehensive guidelines for estimating national greenhouse gas (GHG) emissions and sinks compiled by the Organization for Economic Cooperation and Development (OECD) states that ‘CO2 emissions resulting from bioenergy consumption should not be included in a country’s official emission inventory’ (OECD, 1991) This convention is motivated by the consideration of the carbon (C) neutrality of bioenergy: because growing forests sequester C, as long as areas harvested for biomass are kept forested, the C is again absorbed in growing trees and the net impact on GHG emissions is zero (Manomet, 2010). A GWP equal to 1 is assigned to CO2, which is considered to be r 2011 Blackwell Publishing Ltd offset by the sequestration of the same amount of CO2 that occurred to grow biomass (Reijnders & Huijbregts, 2008; Luo et al, 2009)
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