Thermochemical conversion of microalgal biomass for production of biofuels and co-products
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High oil prices, global warming, and emphasis on renewable technology are attracting new interest in a potentially rich source of biofuels, microalgae. Enormous potential of microalgae for generation of liquid transportation fuels (biodiesel, bioethanol and biocrude/ BioOil) is due to their high growth rate, ability to sequester CO2 and the potential for producing lipids. However, low cell concentration, high harvesting and drying costs, and challenges in production of higher lipid strains are major hurdles in the choice of converting algae biomass into biofuels. In this study, we choose and compare two important thermochemical conversion processes for production of biocrude (BioOil) from a low lipid microalgae: thermochemical liquefaction (TCL) and pyrolysis. TCL is the wet conversion of biomass in hot compressed water whereas pyrolysis is the conversion of dry biomass at moderate temperature and atmospheric pressure. The objectives were to, 1) demonstrate the effect of operating temperature, holding time and solids concentration on product yield and characteristics in TCL process, 2) study the effect of addition of catalysts on biocrude yield and fuel properties, 3) evaluate and compare TCL with a conventional slow pyrolysis process, and 4) recycle the aqueous phase co-product (ACP) from TCL and evaluate it for algae cultivation. TCL and pyrolysis experiments were conducted using batch type reactors and analyses were done by standard laboratory methods. TCL runs were performed at five levels of temperature, holding time, and solids concentration. Catalytic experiments were carried out using Na2CO3, NiO, and Ca3(PO4)2 catalysts at 300-350oC temperature and 30-60 min holding time. Pyrolysis runs were conducted at two temperatures 350oC and 500oC and 60 min holding time. Non-catalytic TCL reported a maximum biocrude yield of ~40%, which was improved to ~51% with Na2CO3 catalyst. Pyrolysis resulted in 23-29% biocrude yield compared to that of 40% by TCL process and even needed more energy than TCL. Biocrude from algae had energy content of ~35 MJ kg-1 which was ~82% of the fossil fuel. Nutrient balance showed that ~75% nitrogen and 29% phosphorous of raw algae were recovered in the ACP by TCL process. With the goal of recycling N and P, experiments were conducted in 250 mL flasks to evaluate the growth of Chlorella minutissima, a wastewater alga using ACP as growth medium. ACP at 500 dilutions reported a biomass growth of 50.5% of the BG11 growth medium. Based on the mass balance an integrated algae biorefinery was proposed for production of biofuels and value added co-products from algae along with carbon sequestration, nutrient recycling and global greenhouse gas reduction based on this study. The results obtained from this research may be used in further research and development of algae biofuels and co-products by the research communities and industries.