An overall analysis of CO2 demand and utilization of microalgal cultures in pilot-scale raceway reactors

Abstract

This study assessed the CO2 demand of microalgae cultures in raceways along the year as a function of environmental and operational conditions, elucidating the most relevant phenomenon taking place in this respect. Experiments were performed using outdoor pilot-scale (80 m2) raceways under different environmental conditions. The results demonstrated that the CO2 demand depends on biomass productivity, which is in turn a function of light availability and temperature. The Biomass productivity varied from 13 to 22 g·m−2·day−1, whereas the CO2 demand varied from 25 to 67 g·m−2·day−1 depending on the environmental conditions. A relevant amount of CO2 is required early in the morning if the pH is not controlled at night. On-demand injection of CO2 represents the major carbon inlet (up to 87 %), whereas most of the carbon leaving the reactor was in the form of biomass (up to 54 %). Carbon losses by decarbonation and with the supernatant were relevant (up to 30 % each) due to the oversaturation of carbon in the culture. Carbon losses into the sump were lower than 10 % thus confirming the adequacy of the design and operation of this section of the raceway. The ratio between the amount of CO2 required and the biomass produced varied from 2.1 to 3.3 g·g−1, suggesting that the utilization efficiency is lower when the biomass productivity is higher. A model for CO2 demand was developed and allowed estimating the CO2 supply required for large-scale systems. The results described in this work highlight the necessity of optimizing the CO2 supply in raceways to minimize biomass production costs.