Lifecycle assessment of microalgae to biofuel: Comparison of thermochemical processing pathways

[Display omitted] •Well to pump environmental assessment of two thermochemical processing pathways.•NER of 1.23 and GHG emissions of −11.4g CO2-eq (MJ)−1 for HTL pathway.•HTL represents promising conversion pathway based on use of wet biomass.•NER of 2.27 and GHG emissions of 210g CO2-eq (MJ)−1 for pyrolysis pathway.•Pyrolysis pathway: drying microalgae feedstock dominates environmental impact. Microalgae is being investigated as a renewable transportation fuel feedstock based on various advantages that include high annual yields, utilization of poor quality land, does not compete with food, and can be integrated with various waste streams. This study focuses on directly assessing the environmental impact of two different thermochemical conversion technologies for the microalgae-to-biofuel process through life cycle assessment. A system boundary of “well to pump” (WTP) is defined and includes sub-process models of the growth, dewatering, thermochemical bio-oil recovery, bio-oil stabilization, conversion to renewable diesel, and transport to the pump. Models were validated with experimental and literature data and are representative of an industrial-scale microalgae-to-biofuel process. Two different thermochemical bio-oil conversion systems are modeled and compared on a systems level, hydrothermal liquefaction (HTL) and pyrolysis. The environmental impact of the two pathways were quantified on the metrics of net energy ratio (NER), defined here as energy consumed over energy produced, and greenhouse gas (GHG) emissions. Results for WTP biofuel production through the HTL pathway were determined to be 1.23 for the NER and GHG emissions of −11.4g CO2-eq (MJ renewable diesel)−1. Biofuel production through the pyrolysis pathway results in a NER of 2.27 and GHG emissions of 210g CO2-eq (MJ renewable diesel)−1. The large environmental impact associated with the pyrolysis pathway is attributed to feedstock drying requirements and combustion of co-products to improve system energetics. Discussion focuses on a detailed breakdown of the overall process energetics and GHGs, impact of modeling at laboratory-scale compared to industrial-scale, environmental impact sensitivity to systems engineering input parameters for future focused research and development, and a comparison of results to literature.