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

Abstract

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-11.4 g CO 2-eq (MJ renewable diesel)-1. WTP biofuel production through the pyrolysis pathway results in a NER of 2.27 and GHG emissions of 210 g CO 2-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.

Lipid content in microalgae determines the quality of biocrude and Energy Return On Investment of hydrothermal liquefaction

“…Lab-scale [12][13][14][15][16][17][18] and pilot-scale [19][20][21] HTL studies using various microalgal strains have produced biocrude yields around 30-50% (dry cell weight basis) regardless of the biochemical composition of microalgae (biomass-agnostic). Since HTL directly treats wet biomass without drying, the entire production chain of microalgal biofuels can be improved in terms of energy balance, economic viability, and environmental impact [22,23]. HTL is therefore gathered considerable attention as a promising unit process for microalgae-based biofuel production.…”

Life Cycle Assessment of Algae Biofuels: Needs and challenges

“…Quinn et al [5] compared two different Well-To-Pump (WTP) thermochemical conversion technologies for the microalgae-tobiofuel process through life cycle assessment: HydroThermal Liquefaction (HTL) and Pyrolysis. NER (Net Energy Ratio) and GHG (GreenHouse Gas emission) were calculated, resulting in 1.23 and 11.4 g CO 2 -eq (MJ renewable diesel) À1 respectively for HTL, and 2.27 and 210 g CO 2 -eq (MJ renewable diesel) À1 for pyrolysis (with algae drying as main limiting factor for algae pyrolysis).…”

Microalgae and Clean Technologies: A Review

“…With the use of this tool, the definition of the goal and scope are established, and it is possible to obtain an inventory of input and output mass and energy through the quantification of generated environmental impacts . For microalgae, many production steps have been studied by LCA, such as cultivation, harvesting/dewatering, and extraction, as have related high‐value products and biofuels, such as biodiesel, biogas, and biojet fuel . After LCA, the conclusion with respect to the defined goals can be discussed to determine the best scenario and understand the impacts associated with each production step …”