A comparative life cycle assessment of microalgae production by CO2 sequestration from flue gas in outdoor raceway ponds under batch and semi-continuous regime

Abstract To be a sustainable energy source, a microalgal cultivation model should address important environmental challenges for its large-scale implementation. In this work, the performance of an outdoor raceway pond system was evaluated for biomass production and sequestration of flue gas carbon-dioxide under different conditions. The microalgae growth was influenced by several physio-chemical factors such as temperature, solar irradiance, carbon-dioxide in the incoming feed, cross-contamination etc. Batch microalgal cultivation, without any external carbon supplementation (0.04% CO2, v/v), exhibited lower areal biomass productivity, whereas flue gas (10 ± 2% CO2, v/v) feeding increased dissolved medium carbon concentrations, which improved carbon-dioxide fixation rate. To further improve the system's performance, semi-continuous regime was adopted that resulted in maximum biomass density of 0.42 g/L and 3.5-fold increase in areal productivity of 11.488 g/m2.s. Next, a “cradle-to-gate” Life Cycle Assessment (LCA) was performed using SimaPro 8.3.0.14 software to identify the potential environmental impacts associated with the production of Chlorella vulgaris biomass (functional unit: 1 kg). Results showed cultivation step to have significant environmental impact (>75%) in terms of GHG emission, energy requirement and other impact categories as estimated by GWP 100 IPCC 2013, CED and ReCiPe Endpoint method. Further, sensitivity analysis assisted in identifying the effect of variables with greatest impact on systems performance in the proposed scenarios; especially with the co-location of cultivation systems and flue gas emitting point sources. The results of attributional LCA revealed that flue gas utilization by microalgae in outdoor raceway ponds has less carbon footprint when a semi-continuous cultivation is carried out and will be more suitable when raceway ponds are co-located with thermal power plants.