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-economicrnassessment (TEA) have been widely applied to the evaluation ofrnhypothetical algae-based biofuel production systems, even thoughrnvery few of these facilities have been constructed and operated as yet.rnThis is both valuable and challenging; since a priori assessmentrnallows for avoidance of sustainability pitfalls but also makes itrndifficult to collect accurate data on process inputs and outputs. Thernexisting LCA literature documents an evolution of expectationsrnrelated to algae biofuels platforms, whereby there has been increasingrnemphasis on making cultivation and conversion operations as simplernas possible. Procurement of low-“cost” or recycled nutrients (mostrnnotably nitrogen and carbon dioxide) is also especially critical, andrnthere is great interest in leveraging existing refining and distributionrninfrastructure. In this vein, the LCA and TEA literature reveal somernconvergence of views on what the most energy-efficient platform forrnalgae biofuels production could be: cultivation in outdoor racewayrnponds, followed by minimal bulk dewatering and conversion into biocrudernvia hydrothermal liquefaction, with internal recycling of thernaqueous co-product to recapture nutrients and valorization of gas andrnsolid-phases to improve the overall energy balance. This approachrnyields energy return on investment (EROI) and greenhouse gasrn(GHG) emissions metrics most closely aligned with desiredrnperformance targets. Perhaps not unsurprisingly, this approach is alsornquite similar to what is currently in operation by the presumed frontrunnerrnamong commercial algae-biofuels producers.rnLCA and TEA have also yielded unexpected conclusions arisingrnfrom sensitivity analyses; whereby it has been possible to assess whatrnindividual inputs or process conditions most strongly impact thernoverall environmental or economic bottom line of a hypothetical fuelproducingrnsystem. This information is valuable for guiding ongoingrnand future experimental work, so that the overall process can bernimproved and optimized. The ultimate goal is production of algaederivedrndistillates (gasoline and diesel) that are certifiable under thernUS’ existing biofuels framework, the Renewable Fuels Standardrn(“RSF2”). Preliminary evidence suggests that this is most likelyrnachievable for the algae HTL pathway noted above.