Mass Balance Modeling and Life Cycle Assessment of Microalgae-derived Biodiesel Production

Biofuels produced from agricultural starch, sugar and oil crops such as corn, sugarcane, and palm, or first-generation biofuels, are produced at commercial scales worldwide. Though most biofuels are produced with the intent to reduce greenhouse gas (GHG) emissions and fossil fuel dependency, these first-generation biofuels have increasingly been shown to be problematic; achieving little to no reduction in GHG emissions compared to their fossil fuel counterparts, competing with food and feed crops, and causing direct and indirect land use change. Second generation biofuel feedstocks, such as microalgae, are hoped to reduce or eliminate the drawbacks of first-generation feedstocks.;This dissertation investigates the environmental impacts of biodiesel production from microalgae, with the main focus on primary energy requirements and life cycle GHG emissions. The dissertation includes a critical review of existing studies; a mass balance model of a simulated microalgae biodiesel production system; a detailed life cycle assessment (LCA) of the production system with a variety of technology options for each step of the production process; and a scenario analysis with alternative utilization scenarios for the primary co-product from the system, lipid-extracted algal biomass residual.;In addition to assessing and informing technology choices and strategies for environmentally preferable pathways among current algal biodiesel technologies, this research also addresses an important methodological issue in LCA, co-product allocation, and proposes some possible solutions to reduce the uncertainty caused by this issue.;Results of the critical review show that significant variation exists among existing LCA studies of algal biodiesel production, which arises from inconsistency in both parameter assumptions and methodological choices. Even after a meta-analysis was conducted, which corrected for some differences in scope and key assumptions, the reviewed studies show a large range in life cycle primary energy and GHG emissions; 0.2 to 8.6 MJ per MJ of algal biodiesel, and -30 to 320 g of CO2e per MJ of algal biodiesel. This range is so large that very little can be concluded regarding the potential for algal biodiesel to meet the goals of second-generation biofuels, and provides the motivation for development an independent and original model for algal biodiesel production.;A mass balance model for an integrated algal oil and biogas system was developed to understand nutrient, water and carbon flows and identify recycling opportunities. The model showed that recycling growth media and recovering nutrients from residual algal biomass through anaerobic digestion can reduce the total demand for nitrogen (N) and phosphorus (P) by 66% and 35%, respectively. Freshwater and carbon dioxide requirements can also be reduced significantly under these conditions.;The mass balance model provided the basis for developing a LCA model capable of incorporating multiple technology options and identifying preferable pathways. The LCA found the best performing scenario consists of normal nitrogen cultivation conditions (as opposed to nitrogen deficient conditions which can increase algal lipid content, but decrease overall productivity), a combination of bioflocculation and dissolved air flotation for harvesting algal cells from cultivation media, centrifugation for dewatering of separated algae, oil extraction from wet biomass using hexane solvent, transesterification of algal oil to biodiesel, and anaerobic digestion of biomass residual with the liquid digestate returning to cultivation ponds. This pathway results in a life cycle energy requirement and GHG emissions of 1.08 MJ and 73 g CO 2-equivalent per MJ of biodiesel, with cultivation and oil extraction dominating energy use and emissions. This result suggests that current technologies can neither achieve a positive net energy return for algal biodiesel, nor achieve substantial reductions in CO2e emissions compared to petroleum diesel.;A comparison between different scenarios for using the major co-product from algae biodiesel production, lipid-extracted algal biomass residual, suggests that utilizing the co-product within the production system for nutrient and energy recovery is preferable than utilizing it outside as animal feed from a life cycle perspective.;A number of possible ways to allocate the environmental burdens between co-products were tested. Among them, system expansion and economic allocation return favorable results compared value-based allocation methods; however, there are still unsolved issues when applying system expansion, for example, current practices do not consider future market values in the context of a consequential LCA.;This dissertation shows that the near-term performance of biodiesel derived from microalgae does not achieve the significant reductions in fossil energy dependence and GHG emissions hoped for from second-generation feedstocks. Furthermore, there is substantial uncertainty in technology performance and other key modeling parameters that could influence these findings. However, some promising, but still uncertain technologies, such as hydrothermal gasification, have the potential to achieve greater reduction in life cycle GHG emissions and energy consumption.