Hydrothermal liquefaction of microalgae biomass cultivated in varied nutrient regimes and species: The energy demand and carbon footprint


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Microalgae are a promising renewable resource with high biomass yields. They can grow in non-potable water and do not compete with the use of agricultural land. The environmental sustainability of processing microalgae biomass in hydrothermal liquefaction (HTL) to obtain biopetroleum and co-products was evaluated using the life cycle assessment technique. The cultivation step in open raceway ponds was considered in scenarios with nutrient availability for high protein content (HP) biomass and with nutrient limitation for high carbohydrate content (HC) and high lipid content (HL) biomass for the species Scenedesmus acutus (SC), Chlorella vulgaris (CV) and Nannochloropsis granulata (NG), followed by harvesting and HTL processing. The biomass productivity in the evaluated scenarios was 18 g (m2 day)-1 on average over the year. The reference flow of 1 kg of microalgae biomass (in total solids) processed in HTL and the biopetroleum yield were used to estimate the results for 1 kg biopetroleum from HTL. The ecoinventTM 3.6 database and assessment methods for cumulative energy demand (CED) in megajoules (MJ) and global warming potential (GWP of IPCC-2021 for 100 years) in kilograms of carbon dioxide equivalent (kg CO2eq) were used in openLCA (R) 1.11.0. The most favorable scenario was NG-HL with the lowest energy demand and carbon footprint, respectively, 19 MJ kg- 1 and 0.85 kg CO2eq kg- 1 of biopetroleum and the highest biopetroleum yield (68 % w/w) with a higher heating value (HHV) of 42 MJ kg- 1. SC-HP achieved the highest energy demand and carbon footprint due to the larger supply of fertilizers. The energy return on investment (EROI in MJ MJCED- 1 ) ranged from 0.8 to 2.2 in the evaluated scenarios. The sensitivity analysis of the cultivation period from early (2 days) to late (7.5 days) harvesting time increased the energy demand by 85 % and the carbon footprint by 70 % in NG-HL. The heat demand in HTL is a key parameter in the energy and environmental performance of biopetroleum production, which ranged from -19 % to 30 % in the sensitivity analysis of the evaluated categories when changing from 5.7 MJ kg- 1 biomass in HTL to 3-10 MJ kg- 1. Therefore, microalgae biomass composition control, nutrient demand, harvesting time and HTL heat should be optimized to increase the competitiveness of HTL biopetroleum on a commercial scale, while reducing the energy demand and carbon footprint.
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Key words
Biotechnology,Biopetroleum,Nitrogen and phosphorus limitation,Microalgae species,Life cycle assessment (LCA),Energy and environmental performance
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