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Biofuels from Liquefaction of Marine Microalgae: Production, Upgrading, and Environmental Impacts
AdvisorHoekman, S. Kent
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Algal biomass is an attractive source for liquid fuel production as it grows rapidly even in adverse conditions, and fixes carbon dioxide from the atmosphere by photosynthesis. Hydrothermal liquefaction (HTL) of microalgae is a promising conversion technology in which algal biomass is treated as an aqueous slurry under high temperature and pressure. Because the HTL process uses the whole biomass, increasing lipid content during cultivation is not essential. With an overall goal to utilize highly productive marine algal strains as feedstocks for liquid hydrocarbon fuel precursors, we investigated the HTL of two marine microalgae (Tetraselmis sp. and Chroocococidiopsis sp.) and one freshwater microalgae (Spirulina) in the presence of co-solvents. The effects of four alcohol-water mixtures [ethanol, isopropanol (IPA), ethylene glycol (EG), and glycerol] as co-solvents were investigated with Tetraselmis sp. at 300 °C and 350 °C. It was shown that each co-solvent mixture affects reaction pressure and product distribution in a different way. The physical properties of the added alcohols have stronger effects on the liquefaction process and product post treatment than do their chemical properties. There is no strong evidence of hydrogen donor behavior of the added alcohols. They (especially isopropanol and ethanol) more likely act as stabilizers to reduce the biocrude viscosity and reduce the rate of viscosity increase as the biocrude ages. It is further shown that increasing the alcohol concentration in the co-solvent mixtures adversely affects the ability to isolate and quantify the HTL products. These concerns would also apply in commercial situations where alcohol solvents are recycled. Based on these results, a preferred approach may be to only use low concentrations of co-solvents (≤10 %) and not attempt their recovery. Effects of operational conditions, including reaction temperature and co-solvent inclusion (IPA and EG) on HTL product yields and compositions were investigated. The highest biocrude yield was obtained from HTL of Spirulina powder at mild reaction conditions (<325 °C). For both marine microalgae that were used, biocrude yield increased with reaction temperature. The biocrude yield increased from 26.3% to 31.0% for Tetraselmis sp., and increased from 17.8 % to 24.7% for Chroocococidiopsis sp. as the reaction temperature was increased from 275 °C to 350 °C. Mild reaction conditions led mainly to lipid extraction, therefore producing relatively high biocrude quality [especially high HHV values (34.3 ± 1.7 MJ/kg) of biocrude produced from Chroocococidiopsis sp. at 300 °C]. Higher reaction temperatures resulted in higher overall biocrude yield, as carbohydrates and proteins begin to contribute to biocrude production under more severe conditions. Addition of 10% IPA as co-solvent promoted a 14.5% and 17.0% increase in biocrude yield for Tetraselmis sp. and Chroocococidiopsis sp., respectively. Gaseous products, which are mainly composed of CO2 (without co-solvent inclusion), could be looped back to the cultivation process to promote algal productivity. With addition of co-solvents, the gaseous products become more fuel rich, and could be used as a heating source for the HTL process. Since over 50% of the algal nitrogen becomes concentrated in the HTL aqueous products, part of the nutrient requirements for algal growth could be satisfied by use of these products. Insoluble products from the HTL process were consistent regardless of the various reaction conditions. The accumulation of phosphorous (P) and calcium (Ca) materials in the insoluble products suggests the possibility of using these solid residues as soil fertilizer. The maximum biocrude yield from Tetraselmis sp. (35.4 %) was achieved at 350 °C when using 10% IPA as co-solvent. However, this biocrude also contained significant amounts of nitrogen and oxygen. To further reduce these heteroatoms, biocrude obtained from HLT of Tetraselmis sp. with and without co-solvent inclusion were catalytically hydrotreated under high pressure hydrogen with three different catalysts. To compare the overall performance of fuels derived from HTL processing of marine microalgae with other fuels, analysis of energy recovery in the biocrude is not sufficient. To address this, life cycle analyses (LCA) of microalgae to biofuel conversion pathways were performed for two HTL scenarios: lab-scale and field-scale. The lab-scale scenario was based on experimental data obtained in our own lab. The field-scale scenario was representative of more realistic commercial processes and included experimental data from a demonstration facility. Both scenarios suggested that algal biofuels can achieve significant reduction in GHG emissions compared to conventional petroleum alternatives, but have similar GHG emissions compared to terrestrial biomass-derived counterparts. A viable energy return on investment (EROI) was calculated when algae were processed at lab-scale using a coagulation-flocculation harvesting method. For the field-scale scenario, EROI was near to 1 and comparable to other conventional terrestrial biofuels. Overall, this dissertation research suggests that hydrothermal treatment of marine microalgae can be a sustainable processing technique for production of biofuels and chemicals.