New Life Cycle Analysis Finds that Current Technology for Producing Algal Biodiesel Can Jeopardize the Overall Energetic Balance; Oil Extraction Technique Is Key
|Total energetic debt of 1 MJ of biodiesel and its distribution within the production chain. According to the study, only the wet extraction on low-N grown algae has a positive balance. Credit: ACS. Click to enlarge.|
Biodiesel production from microalgae, while looked upon as an environmentally advantageous source of fuel, suffers from several drawbacks at the current level of technology that can jeopardize the overall energetic balance, according to a life cycle analysis (LCA) by a team of researchers in France. An open-access paper on their work was published online 27 July in the ACS journal Environmental Science and Technology.
Despite strong interest and investment in development, there is as yet no industrial-scale production of biodiesel from microalgae, the researchers noted, and hence no thorough LCA of the production chain from culture to fuel is currently available. The key objective of the study was not to offer a LCA of current microalgal biodiesel technology, the authors wrote, but to identify the obstacles and limitations which should receive specific research efforts to make this process environmentally sustainable.
For the study, the team extrapolated laboratory observations combined with known processes developed for first generation biofuel to design what they termed a realistic virtual industrial facility. They analyzed two different culture conditions (nominal fertilizing or nitrogen starvation) as well as two different extraction options (dry or wet extraction). For the algae strain, they used Chlorella vulgaris. The best scenario was compared to first generation biodiesel and oil diesel.
The team performed a cumulative energy analysis to analyze the total energetic debt of 1 MJ of biodiesel and its distribution within the production chain (see chart above). Cumulative Energy Demand (CED) includes energy used at the facility but also energy required for the production of the required inputs (fertilizers) and construction of infrastructure buildings.
When taking into account all the energetic debt of the process chain, it appears that only the wet extraction on low-N grown algae has a positive balance. Other scenarios lead to negative energetic balance despite a 100% energy extraction from the oilcake. It can also be noticed that the application of a nitrogen stress improves the CED by 60% whereas CED is only increased by 25% with the wet extraction. Obviously low-N culture has lower fertilizer requirements but also implies a lower drying and extraction effort while the wet extraction needs a larger initial production due to its lower extraction yield.
...Energetic balance of biodiesel production from microalgae shows that it can be rapidly jeopardized ending up with a counter-productive production chain. Whereas production of fuel differs slightly from the simple production of energy (production of a storable product useable in automotive engine requires specific properties), it is mandatory to have at least positive energetic balance.—Lardon et al.
Their analysis showed that any improvement of oil extraction technique would have a direct impact on the sustainability production— 90% of the process energy consumption is dedicated to lipid extraction (70% when considering the wet extraction). They concluded that specific research must investigate new processes in lipid recovering with limited drying of the biomass. They also found that in comparison to conventional energetic crops, high photosynthetic yields of microalgae significantly reduce land and pesticide use—but not fertilizer needs.
They suggested several improvements which could contribute to reduce most of the impacts. of large-scale production:
- The choice of microalgal species maintaining high lipid and low protein contents with sustained growth rates (e.g., low-N culture, strain selection, or modification);
- The setup of an energetically efficient extraction method; and
- The recovery of energy and nutrients contained in the oilcake.
More generally, LCA appears as a relevant tool to evaluate new technologies for energy production. Even when dealing with young and immature technologies, this tool identifies the technological bottlenecks and therefore supports the ecodesign of an efficient and sustainable production chain.—Lardon et al.
Laurent Lardon, Arnaud Hlias, Bruno Sialve, Jean-Philippe Steyer and Olivier Bernard (2009) Life-Cycle Assessment of Biodiesel Production from Microalgae. Environ. Sci. Technol., Article ASAP doi: 10.1021/es900705j