A recent report by the German Academy of Sciences Leopoldina (discussed here on the 10th of august by Diederik van der Hoeven) argues in favour of selective energetic usage of biomass. That is good advice. Photosynthesis is beautiful but its energetic efficiency is modest. Corrected for inputs of fossil fuels, the conversion efficiency of solar energy for Northwest European rapeseed-biodiesel into automotive kilometres is 0.01%. For the combination of an electric car and solar cells in Northwestern Europe the corresponding solar energy conversion efficiency is about 5% (*). This is a difference of roughly a factor 500.
According to van der Hoeven, the Leopoldina report is positive as to the energetic usage of agricultural residues. It is uncertain whether such usage would be a good thing. Average European agricultural land suffers from a depletion of organic carbon. This leads to among other things increasing erosion and increasing losses of nutrients and may ultimately cause lower fertility. An obvious response to this problem would be an increased return of agricultural residues to European agricultural soils. The compatibility of increased energetic usage of agricultural residues with the need to stop the depletion of organic carbon in European agricultural soils is, given available research, highly uncertain.
Biochemicals and biobased polymers
In his contribution of the 10th of august Diederik van der Hoeven raised the question: what about chemicals from biomass? This question was not dealt with in the Leopoldina report. There has been substantial comparative life cycle assessment of biomass-based chemicals and conventional chemicals to be used in the production of plastics (**). Bioplastics such as polylactic acid (PLA) and polyhydroxyalkanoate (PHA) are already marketed. The results of the life cycle assessments show that the environmental burdens of currently available PHA and PLA (including Nature Works PLA) are larger than the environmental burdens of the major fossil-fuel based plastics polyethylene and polypropylene. Both the environmental burden of agriculture and the environmental burden of converting biomass into bioplastic are relatively large. If the environmental burden of agriculture is eliminated from the assessment by using corn stover as feedstock for the production of PHA the environmental burden of PHA is still larger than the environmental burden of polyethylene and polypropylene. However, the environmental burden of the bioplastics PHA and PLA is lower than the environmental burden of PET or polycarbonate produced on the basis of fossil fuel-derived chemicals. These findings also favour selective usage of biomass. In view of their environmental burden, ‘biodegradable’ bioplastics merit only application when there is no other option. And bioplastics are currently environmentally no match for applications of polyethylene and polypropylene which can be reused many times (such as plastic crates for beer).
(*) L. Reijnders, M.A. J. Huijbregts, Transport Biofuels: A Seed to Wheel Perspective, Springer, London, 2009
(**) M.A. Tabone, J.L. Clegg, E.J. Beckman, A. Landis, Sustainability metrics: life cycle assessment and green design in polymers, Environmental Science & Technology 44 (2010) 8264-8269