Life Cycle Energy and Carbon Footprints of Microalgal Biodiesel Production in Western Australia: A Comparison of Byproducts Utilization Strategies

Abstract

This study compares the performances of anaerobic digestion and hydrothermal liquefaction as byproducts (defatted microalgae and glycerol) utilization strategies to offset overall life cycle energy and carbon footprints of microalgal biodiesel production in Western Australian (WA). Utilization of byproducts via anaerobic digestion or hydrothermal liquefaction enables the production of electricity and process heat, as well as the recovery of inherent nutrients. As a result, the anaerobic digestion route and hydrothermal liquefaction route substantially reduce life cycle energy inputs for producing 1 MJ biodiesel from 4.3 MJ (without byproducts utilization) to 1.3 and 0.7 MJ, yielding carbon footprints of ∼80 and ∼33 g CO2-eq/MJ biodiesel, respectively. The results indicate that hydrothermal liquefaction, which shows better life cycle performance and requires smaller reactor footprint than anaerobic digestion, can be another potential strategy to recover energy embedded in defatted microalgae. It is also evident that while vast coastal areas are available in WA for marine microalgae cultivation, further technological advances are required to realize a truly sustainable biodiesel production from microalgae. Sensitivity analyses suggest that key R&D areas are improvement of microalgae biological properties (e.g., growth rate and lipid content) and innovations in engineering designs (e.g., culture circulation velocity, methane yield during anaerobic digestion, and bio-oil yield during hydrothermal liquefaction).