12 1 Pierre Collet1, Daniele Spinelli2, Laurent Lardon1, Arnaud

Helias1’3 , Jean-Philippe Steyer1, Olivier Bernard4

1INRA UR0050, Laboratoire de Biotechnologie de l’Environnement,

Avenue des Etangs, Narbonne, France
2Department of Chemistry and Center for Complex System Investigation,
University of Siena, Siena, Italy
3Montpellier SupAgro, Montpellier, France
4INRIA BIOCORE, Sophia Antipolis Cedex, France

13.1 INTRODUCTION

Environmental impacts and depletion of fossil energies have promoted the development of alternative and renewable sources of energy. Nonetheless, it is clear now that the replacement of current fossil energy will require both the development of new strategies to reduce our global energy consumption and the development of a panel of renewable energy sources. Re­newable energy can be extracted from solar, wind, or geothermal energy. However, these en­ergy forms are globally hard to store and hence cannot yet replace our consumption of fossil fuel for some important functions, such as powering cars and planes.

So far, several paths have been explored to produce fuels from renewable sources, the most developed strategies leading to the production of so-called first — and second-generation biofuels. First-generation biofuels are based on fuel production (ethanol or methylester) from a currently cultivated and harvested biomass (e. g., corn, rapeseed). Second-generation biofuels correspond to the development of new energy production pathways from usual feed­stock not reclaimed by food production (e. g., straw or wood). The development of first- generation biofuels has been criticized—first, because of the direct competition they create with food crops in a context where food security is a raising concern, and second, because of their actual poor environmental performance. Indeed, inputs to production (e. g., fertilizer or pesticides), feedstock culture, harvest, and transformation imply fuel consumption and lead to new pollutant emissions (Borjesson and Tufvesson, 2011), especially emissions of greenhouse gases to the environment (Searchinger et al., 2008). Consequently, energy and en­vironmental benefits of these biofuels are limited. Second-generation biofuels improve envi­ronmental performance but are not free of criticisms.

These observations advocate for the necessity of systematically assessing new energy pro­duction paths with a life-cycle assessment (LCA) perspective, which means, first, the adoption of a cradle-to-grave perspective, that is, looking at resource depletion, energy consumption, and substance emissions of all the processes required to achieve the production and use of the fuel, and second, the assessment of several environmental impacts, not only global warming potential or net energy production. LCA is an ISO method developed with this aim: It allows the detection of pollution transfer from one step to another or from one kind of environmental impact to another. The European Directive on Renewable Energy (European Union, 2009), adopted in 2009, embraces LCA as a reference method to assess environmental impacts of biofuel and to meet greenhouse gas reduction objectives of 50% in 2017.

Third-generation biofuels correspond to the development of bioenergy productions based on new feedstock reputed to have lower land competition. Microalgae belong to this third category. Their very high actual photosynthetic yield and their ability to accumulate lipids, or, for some species, starch, added to the possibility that they can be cultivated in controlled environments, promise the potential of biofuel that has a low competition with food crops (Chisti, 2007) and limited environmental impacts. In addition, the ability to use CO2 directly from industrial emissions as a source of carbon for the growth of microalgae is a promising feature for flue-gas mitigation (Huntley and Redalje, 2007; Chisti, 2007). However, this promise should be challenged. So far, microalgae industrial production has been developed only for the production of high-value molecules (such as beta-carotenes) or dietary supplements (Spirulina or Chlorella can be found as pills in health shops); hence, energy or environmental performance has never been a concern. Moreover, the scale of the existing facilities is far smaller than that required for fuel pro­duction (at least several hundred ha). It is necessary to assess the expected environmental performance of these potential production systems in order to detect technological bottle­necks and to determine which processes should be optimized in priority. This approach is now necessary to design a sound, energetically efficient, and environmentally friendly bio­fuel production system.

Since the new focus of international scientific and economic communities on microalgae — based biofuel, many environmental, energy, or economical assessments have been published, with different final energy carriers or different production assumptions. Here we propose to review a set of publications, all of them published in peer-reviewed scientific journals, using the LCA method to assess the environmental impacts linked to microalgae-based biofuel. The lack of real industrial facilities dedicated to energy production from microalgae imposes the use of models and extrapolations to describe the production systems. In addition, system frontiers and coproduct management differ among the studies. Altogether, this leads to di­vergences between publication results. This review aims to identify and explain this variabil­ity and then to propose guidelines to improve future LCAs of algal-based bioenergy production systems. This work is a mirror of this diversity and underlines the difficulty in comparing different studies without common assumptions.