ALGAE BIOFUEL PRODUCTION STRATEGIES: WHAT HAVE WE LEARNED FROM LCA AND TEA AND WHAT DOES IT MEAN?

摘要

Life cycle assessment (LCA) and techno-economic assessment (TEA) have been widely applied to the evaluation of hypothetical algae-based biofuel production systems, even though very few of these facilities have been constructed and operated as yet. This is both valuable and challenging; since a priori assessment allows for avoidance of sustainability pitfalls but also makes it difficult to collect accurate data on process inputs and outputs. The existing LCA literature documents an evolution of expectations related to algae biofuels platforms, whereby there has been increasing emphasis on making cultivation and conversion operations as simple as possible. Procurement of low-"cost" or recycled nutrients (most notably nitrogen and carbon dioxide) is also especially critical, and there is great interest in leveraging existing refining and distribution infrastructure. In this vein, the LCA and TEA literature reveal some convergence of views on what the most energy-efficient platform for algae biofuels production could be: cultivation in outdoor raceway ponds, followed by minimal bulk dewatering and conversion into bio-crude via hydrothermal liquefaction, with internal recycling of the aqueous co-product to recapture nutrients and valorization of gas and solid-phases to improve the overall energy balance. This approach yields energy return on investment (EROI) and greenhouse gas (GHG) emissions metrics most closely aligned with desired performance targets. Perhaps not unsurprisingly, this approach is also quite similar to what is currently in operation by the presumed front-runner among commercial algae-biofuels producers.