CO2 from industrial off-gases for algae cultivation

In this work, technical solutions for capturing CO2 from CO2-containing off-gases from industry for feeding an algal cultivation were qualitatively evaluated. Also, cultivation of algae using both vent gases from a sour gas processing plant and flue gases from a coal-fired combined heat and power (CHP) plant was studied. The most promising methods for CO2 capture seem to be those that absorb CO2 directly into the cultivation media by using separate bubbling carbonation columns, both for open ponds and closed photobioreactors. This lowers the energy requirements in comparison to flue gas injection and also enables the remainder of the flue gas to be led out through the existing flue gas stack. The low capacity of water to dissolve CO2 can be improved by addition of alkaline salts. The growth of two green algae, one diatom, and one cyanobacterium was examined in a laboratory-scale, batch-mode comparative cultivation experiment, using both pure CO2 and flue gas from a coal-fired CHP plant. No significant statistical differences in the growth were observed between the experiments except for the cyanobacterium, which had a decreased growth during flue gas cultivation. Microalgae suitable for cultivation using vent gases from a sour gas processing plant were screened by employing a 20 L photobioreactor. Based on these experiments, a certain mixture of microalgae exhibited rapid growth and better tolerance towards in terms of time taken to reach pH 7. A small-scale CO2 capture and cultivation pilot was set up using a 0.3 m3 CO2 absorption column for absorbing CO2 from vent gas in connection to a 0.2 m3 raceway pond. The produced algae was harvested and sent for anaerobic digestion studies. The experiments were successful, with a microalgae yield of 18 g/m2/day achieved, which on anaerobic digestion yielded about 0.4 m3 CH4/kg volatile solids fed.