274452 Evaluating Environmental Sustainability of Microalgal Biofuels: A Life Cycle Thermodynamic View
Renewable fuels represent an important area for research due to increasing global energy demand, oil price and supply volatility, energy import reliance and security, as well as concerns over resource depletion and environmental impacts. Transportation fuels with reduced resource utilization and environmental impacts are of particular interest as the transportation sector represents 28% of overall U.S. primary energy consumption. Further motivation is provided by the Energy Independence and Security Act of 2007 (EISA), which mandates the domestic production and use of 21 billion gallons of cellulosic and advanced biofuels by 2022. Microalgae represent a promising feedstock for next-generation biofuels due to their high growth rates, high lipid content and rapid harvest cycle. In addition, algae production does not exert direct market pressure on food crop prices and can utilize marginal lands, resulting in reduced land-use change impacts. Much of microalgal biofuels research focuses on either open-pond or photobioreactor cultivation systems, coupled with a variety of harvesting and oil-extraction schemes, and culminating in the production of biodiesel via transesterification along with glycerin and large quantity of residual biomass. Less attention has been placed on alternative conversion technologies and fuel products, or on scalable uses for the residual biomass and other co-products. Renewable diesel (RD) is attractive as a fuel product because of its high energy density and Cetane number, as well as its good cold-flow properties and storage stability. Also referred to as “green diesel” or “hydrotreated vegetable oil,” RD is produced by hydrotreating bio-oils to remove oxygen molecules and to fully saturate double bonds. Holistic, comparative evaluation of emerging algae-to-fuel systems considering resource consumption, emissions and their impact across the entire life cycle is critical for ensuring the long-term sustainability of emerging algae-based energy systems.
Much of the existing work on life cycle assessment (LCA) of microalgae has focused on quantifying the life-cycle greenhouse gas (GHG) emissions and the net energy balance for cultivation and biofuel production. Net energy analysis suffers from several limitations such as ignoring the differences in energy quality of different resources and accounting only for non-renewable energy sources. Such aggregation without attention to the quality of resources has led some researchers to question the utility of resulting metrics. Thermodynamic-based methods have been suggested to address the limitations of traditional LCA and to better quantify resource utilization in biofuel production systems. This work proposes to integrate traditional LCA with exergy-based thermodynamic analysis to better compare the sustainability of biofuel systems. Exergy, or available work, represents a common metric for evaluating the quality of energy carriers based not on gross energy content, but on ability to do work. Defined as the maximum work a system can produce if it is brought to thermal, chemical, and mechanical equilibrium with its reference environment, exergy represents the maximum useful work potential of a system. Exergy analysis also serves to aggregate and compare material flows with energy flows of differing qualities using one common unit (Joules). Recent studies into the exergetic life cycle assessment (ELCA) of biofuels consider the transesterification of waste oils to produce biodiesel, however no complete exergy-based assessment of the microalgal-derived biodiesel and renewable diesel production chains is available.
This study undertakes a well-to-pump traditional LCA and exergetic analysis of renewable diesel and biodiesel fuels derived from algae cultivated in open ponds. Traditional LCA methodology is first used to examine resource consumption and environmental impacts, focusing in particular on net-energy balance and GHG emissions of the two systems. Preliminary LCA results for both fuels indicate a net negative energy balance with fossil energy ratios ranging from 0.26-1.26 depending on cultivation variables, processing technologies and biomass composition. Fuel upgrade technology choice is found to have a comparatively small contribution to overall life-cycle energy use and emissions. GHG emissions are comparable for both fuel conversion pathways, but vary strongly with the chosen co-product allocation methodologies.
For the exergetic LCA, various techniques and metrics are used at multiple scales, providing irreversibility-based measures for quantifying the consumption, use efficiency and depletion of material and energy resources. Quantifying exergy losses at the process scale indicates thermodynamic irreversibilities (entropy generation). This results in a simple exergetic efficiency factor for each process, providing insights for where to minimize exergy losses to improve efficiency across the entire production chain. A variety of hierarchical sustainability metrics spanning the detailed data level to aggregate metrics based on thermodynamics are developed in this work. This includes measures such as exergetic return on investment (ExROI), renewability index, exergy breeding factor, and life cycle exergetic efficiency for microalgal biofuels. Uncertainty and sensitivity to changes in key parameters including the biomass composition, co-product options, and alternative processing technologies will be discussed. Such insights are essential in determining which biofuel products and processes are most sustainable and best suited for development at industrial scale. The research will result in a more holistic life-cycle evaluation of the considered biofuels and will address trade-offs between process technologies, fuel products and co-products, and environmental impacts at both the life-cycle and process scales.
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