George G. Zaimes

Microalgal biomass production pathways: evaluation of life cycle environmental impacts

BackgroundMicroalgae are touted as an attractive alternative to traditional forms of biomass for biofuel production, due to high productivity, ability to be cultivated on marginal lands, and potential to utilize carbon dioxide (CO2) from industrial flue gas. This work examines the fossil energy return on investment (EROIfossil), greenhouse gas (GHG) emissions, and direct Water Demands (WD) of producing dried algal biomass through the cultivation of microalgae in Open Raceway Ponds (ORP) for 21 geographic locations in the contiguous United States (U.S.). For each location, comprehensive life cycle assessment (LCA) is performed for multiple microalgal biomass production pathways, consisting of a combination of cultivation and harvesting options.ResultsResults indicate that the EROIfossil for microalgae biomass vary from 0.38 to 1.08 with life cycle GHG emissions of −46.2 to 48.9 (g CO2 eq/MJ-biomass) and direct WDs of 20.8 to 38.8 (Liters/MJ-biomass) over the range of scenarios analyzed. Further anaylsis reveals that the EROIfossil for production pathways is relatively location invariant, and that algae’s life cycle energy balance and GHG impacts are highly dependent on cultivation and harvesting parameters. Contrarily, algae’s direct water demands were found to be highly sensitive to geographic location, and thus may be a constraining factor in sustainable algal-derived biofuel production. Additionally, scenarios with promising EROIfossil and GHG emissions profiles are plagued with high technological uncertainty.ConclusionsGiven the high variability in microalgae’s energy and environmental performance, careful evaluation of the algae-to-fuel supply chain is necessary to ensure the long-term sustainability of emerging algal biofuel systems. Alternative production scenarios and technologies may have the potential to reduce the critical demands of biomass production, and should be considered to make algae a viable and more efficient biofuel alternative.

Biofuels via fast pyrolysis of perennial grasses: a life cycle evaluation of energy consumption and greenhouse gas emissions.

A well-to-wheel (WTW) life cycle assessment (LCA) model is developed to evaluate the environmental profile of producing liquid transportation fuels via fast pyrolysis of perennial grasses: switchgrass and miscanthus. The framework established in this study consists of (1) an agricultural model used to determine biomass growth rates, agrochemical application rates, and other key parameters in the production of miscanthus and switchgrass biofeedstock; (2) an ASPEN model utilized to simulate thermochemical conversion via fast pyrolysis and catalytic upgrading of bio-oil to renewable transportation fuel. Monte Carlo analysis is performed to determine statistical bounds for key sustainability and performance measures including life cycle greenhouse gas (GHG) emissions and Energy Return on Investment (EROI). The results of this work reveal that the EROI and GHG emissions (gCO2e/MJ-fuel) for fast pyrolysis derived fuels range from 1.52 to 2.56 and 22.5 to 61.0 respectively, over the host of scenarios evaluated. Further analysis reveals that the energetic performance and GHG reduction potential of fast pyrolysis-derived fuels are highly sensitive to the choice of coproduct scenario and LCA allocation scheme, and in select cases can change the life cycle carbon balance from meeting to exceeding the renewable fuel standard emissions reduction threshold for cellulosic biofuels.

Multistage torrefaction and in situ catalytic upgrading to hydrocarbon biofuels: analysis of life cycle energy use and greenhouse gas emissions

A well-to-wheel life cycle assessment (LCA) model is developed to characterize the life cycle energy consumption and greenhouse gas emissions profiles of a series of novel multistage torrefaction and pyrolysis systems for targeted thermochemical conversion of short rotation woody crops to bio-oil and in situ catalytic upgrading to hydrocarbon transportation fuels, and to benchmark the results against a base-case fast pyrolysis and hydrodeoxygenation (HDO) platform. Multistage systems utilize a staged thermal gradient to fractionate bio-oil into product streams consisting of distinct functional groups, and multi-step chemical synthesis for downstream processing of bio-oil fractions to hydrocarbon fuels. Results at the process scale reveal that multistage systems have several advantages over the base-case including: (1) ∼40% reduction in process hydrogen consumption and (2) the product distribution for multistage systems are skewed towards longer carbon chain compounds that are fungible with diesel-range fuels. LCA reveals that the median Energy Return On Investment (EROI) and life cycle greenhouse gas (GHG) emissions for multistage systems range from 1.32 to 3.76 MJ-fuel/MJ-primary fossil energy and 17.1 to 52.8 gCO2e/MJ-fuel respectively, over the host of co-product scenarios and allocation schemes analyzed, with fossil-derived hydrogen constituting the principle GHG and primary energy burden across all systems. These results are compelling and indicate that multistage systems exhibit comparatively higher gasoline/diesel-range fuel yield relative to current technology, and produce a high quality infrastructure-compatible hydrocarbon transportation fuel capable of achieving over 80% reduction in life cycle GHG emissions relative to baseline petroleum diesel.

Assessing the critical role of ecological goods and services in microalgal biofuel life cycles

Microalgal bioenergy systems are gaining attention as a commercial biotechnical platform for producing renewable transportation fuels. In recent years, process-based life cycle assessment (LCA) has been extensively applied to understand the life cycle environmental impacts of emerging microalgal biofuel systems. However, conventional process-based LCA fails to account for the role of ecological goods and services within fuel and product life cycles. Additionally, traditional life cycle energy analysis suffers from several limitations such as ignoring the difference in quality and substitutability of resources, and accounting for only the first law of thermodynamics. To address these shortcomings, a hybrid Ecologically based-LCA (EcoLCA) model is developed to quantify the contribution of ecological resources within the algae-to-fuel supply chain and to compare the resource intensity of producing microalgal derived renewable diesel (RD) to that of petroleum diesel (PD). Multiple thermodynamic return on investment (ROI) metrics and performance indicators are used to quantify the consumption of ecological goods and services, environmental impacts, and resource intensity of producing microalgal RD. Results indicate that the quality corrected thermodynamic return on investment and renewability index for microalgal RD ranges from 0.17 to 0.44 and 3.51% to 6.36% respectively, depending on the choice of coproduct options and processing technologies. This work reveals that algae-to-fuel systems are highly dependent on non-renewable ecological resources reflected in their low renewability index; have a low quality corrected thermodynamic ROI (<1) and thus are not energetically viable; and are more ecologically resource intensive as compared to their petroleum equivalent—potentially negating their environmental benefits.

Integrating LCA and thermodynamic analysis for sustainability assessment of algal biofuels: Comparison of renewable diesel vs. biodiesel

Advanced biofuels are attracting intense interest from government, industry and researchers as potential substitutes for petroleum gasoline and diesel transportation fuels. Microalgaes advantages as a biofuel feedstock are due particularly to their rapid growth rates and high lipid content. Several life cycle analysis (LCA) studies have been conducted on the production of biodiesel, however less attention has been paid to algae-derived green diesel (renewable diesel II), a promising alternative fuel product. Renewable diesels advocates suggest that it has superior energy density, shelf stability and can function as a drop-in replacement for petroleum diesel due to their similar chemical composition and fuel properties. Fewer studies have attempted to quantify the sustainability of algae-derived renewable diesel, though renewable diesel options are examined in the current GREET model. This study conducts a well-to-pump LCA focusing on this Renewable Diesel II (RD2) upgrade pathway and comparing it with the corresponding pathway from algal biomass to biodiesel. Particular attention is paid to primary energy use and fossil energy ratio (FER), greenhouse gas emissions, and an initial investigation of thermodynamic metrics. While hydrotreating is less than half as energy intensive a fuel upgrade process as transesterification, the overall life-cycle energy consumption and greenhouse gas emissions are found to be nearly equal for renewable diesel and biodiesel. The complete biofuel production process is only found to be net energy positive for scenarios with reduced burdens from both CO2 sourcing and biomass drying.

Life cycle sustainability aspects of microalgal biofuels

Holistic systems analysis of biofuel production that considers the environmental impacts over the entire fuel life cycle, from raw material extraction to cultivation, conversion, and final use, has emerged as the leading methodological paradigm for quantifying the environmental sustainability of biomass-to-fuel systems. This chapter reviews the application of life cycle assessment (LCA) for environmental evaluation of emerging microalgal biofuels. Several prominent environmental performance indicators including carbon footprint, energy return on investment, and water footprint are reviewed in the context of microalgal systems and other leading biofuels/biofeedstocks. A case study of microalgal biodiesel production is provided in the text. The results are compared with previous microalgae LCAs and discussed in a broader environmental sustainability context.

Integrating the Role of Thermodynamics in LCA: A Case Study of Microalgal Biofuels

Liquid transportation fuels derived from biomass have gained considerable attention due to their potential to reduce the carbon intensity in the transportation sector and mitigate the effects of global climate change. As such, holistic assessment of the environmental impacts throughout the emerging biofuel supply chains is critical for determining the sustainability of emerging biofuel pathways. Systems analysis via life cycle assessment (LCA) has emerged as the prevalent methodological framework for quantifying the environmental impacts of biofuel systems. However, traditional LCA fails to account for the role of ecosystem goods and services in fuel and product life cycles. In addition, LCA and energy analysis suffers from several limitations such as ignoring the differences in the quality of the energy resource, assuming perfect substitutability of resources, and accounting for only the first law of thermodynamics. Thermodynamic based approaches that utilize exergy and emergy can address several limitations of traditional LCA. This chapter discusses the research progress and methodological advances in thermodynamic based methods for LCA and its application in assessing the environmental impacts of emerging microalgal biofuels. An analysis of microalgal biofuel production is considered as a case study, and the results of thermodynamic metrics are benchmarked against other life cycle based environmental sustainability metrics.