Microalgae to biofuels lifecycle assessment - Multiple pathway evaluation

A variety of researchers have constructed and presented lifecycle assessments of the microalgae-to-biofuel process, however, inconsistencies in system boundary definitions and high-level process modeling have led to a wide range of results. This study integrates engineering process models validated through experimental and modeling research to perform an environmental assessment of four microalgae-to-biofuel production scenarios leveraging the Argonne National Laboratory GREET model. The baseline scenario consists of a down flow open pond growth system, three phase de-watering step (settling, dissolved air flotation, and a centrifuge), hexane extraction and nutrient recovery using anaerobic digestion. The net energy ratio (NER), defined as energy consumed over the produced energy, and greenhouse gases (GHG) for the baseline scenario are 0.7 MJ MJ− 1 and − 41.7 g CO2-eq MJ− 1 respectively. Three alternative scenarios are also evaluated: 1) Improved microalgal productivity, 2) supercritical CO2 extraction, and 3) no nutrient recycle. This research shows that supercritical CO2 extraction is neither currently energetically- nor environmentally favorable and that nutrient recycle plays an integral role in achieving favorable NER and GHGs. The study highlights on the systems level, two findings related to the NER; 1) the NER is minimally impacted with increased productivity and 2) increasing microalgae lipid content detrimentally affects the NER which is attributed to the reduction in the total energy that can be captured by the anaerobic digester.