Environmental Impact Assessment of Microalgae to Biofuel Utilizing Thermochemical Processing

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Jason Quinn


Microalgae are currently 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 waste streams. This study focuses on directly assessing the impact of two different thermochemical conversion technologies on the microalgae to biofuel process through life cycle assessment. A system boundary of a cradle to pump is leveraged and sub 'process models are developed to be representative of a large-scale microalgae to biofuel process including growth, dewatering, drying, bio-oil recovery, bio-oil stabilization, and conversion to renewable diesel. Two different thermochemical bio-oil conversion systems are modeled and compared, pyrolysis and hydrothermal liquefaction (HTL). Both technologies having the capability of improving yields through the conversion of non-lipid constituents to bio-oil. Systems engineering models were experimentally validated and integrated with life cycle modeling to determine the environmental impact of the various pathways. Metrics of net energy ratio (NER) defined here as energy consumed over energy produced and global warming potential (GWP) are used to directly quantify and compare the modeled process to conventional fuels. Biofuel production through HTL was determined to be the more favorable process with a NER of 1.42 and GHG emissions of -1.1 g CO2 eq (MJ renewable diesel)-1 compared to pyrolysis with a NER of 2.40 and GHG emissions of 272.6 g CO2 eq (MJ renewable diesel)-1. Results include a detailed breakdown of the overall process energetics and GWP for the HTL and pyrolysis processing pathways, impact of modeling at laboratory- scale compared to large-scale, and environmental impact sensitivity to systems engineering input parameters.

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