13.6.1 Perimeter and Functional Units

The lack of inclusion of biofuel combustion from the perimeter of the study can facilitate the comparison between different technologies or energy production pathways, but it ham­pers the assessment of the real carbon balance; indeed, some of the carbon atoms of the methylester stem from methanol, which is usually produced from fossil fuel (Stephenson et al., 2010). Moreover, it ignores environmental impacts from combustion (such as photo­chemical oxidation and particulate matter formation). Finally, all engines do not have the same efficiency, and hence a fair comparison should be based on the available work produced by the use of the fuel rather than on the chemical property of the fuel only.

To harmonize LCA results and provide a better basis for comparisons, the energy content of intermediate products (raw algae, oil, oil extraction residues, and methylester) should be systematically provided and justified. We also recommend using the LHV instead of the HHV; indeed, in most cases, biofuel will be used in engines (internal combustion engines or turbines) that are unable to use the energy stored in the water vapor resulting from fuel combustion.

As shown in the preceding section, the choice of using allocation or substitution to handle the multifunctionality of processes has a strong influence on the results. Even though the sys­tem expansion is a priori preferable, it can lead to an increase in the overall uncertainty when performance of substituted processes are little known (performance of anaerobic digestion of oilcakes) or if the validity of the substitution is questionable (use of oilcake extraction as an­imal food, for instance).