13.6.3.1 Climate Change and Consideration of Biogenic Carbon

An important point in the assessment of greenhouse gases is the consideration of the fix­ation of CO2 during photosynthesis in the cultivation step and the emissions of CO2 during the combustion step (if this last step is included in the perimeter of the study). In the publi­cations of Batan et al. (2010) and Clarens et al. (2010), the fixed CO2 is negatively counted in the global balance of the greenhouse gas (respectively, -75.3 g CO2 eq MJ-1 and -69.4 g CO2 eq MJ-1). But this CO2 is then emitted to the atmosphere during the combustion step. This emission is considered by Batan et al. (2010) but not by Clarens et al. (2010), so in this last case the production of bioenergy from microalgae is a sink of carbon, and the greenhouse balance is widely underestimated. In most of the LCA studies, fixation and then emission of biogenic carbon in the atmosphere are considered neutral processes from a "climate-change" point of view. Consequently, most of the authors do not count the fixation of the CO2 during the cultivation step nor the emission during the combustion step. We recommend dedicating specific attention to this point to guarantee a sound carbon balance.