Uptake of carbon dioxide from flue gas by microalgae

The technology under development seeks to provide affordable, and efficient methods for reducing carbon dioxide and other emissions from coal-fired electricity generation based on biological assimilation. Photosynthetic microorganisms such as microalgae, when grown in large outdoor ponds could use carbon dioxide from flue gas directly injected into the culture. This process requires land, water, sunlight and other nutrients. Currently, commercial production of photosynthetic microorganisms is used to produce high value products such as pigments. This work seeks to extend this technology to the energy field. Our results to date indicate that at least some microalgae can tolerate moderate levels of SOx and NOx in laboratory culture, and that a well-engineered outdoor pond can easily achieve in excess of 90% carbon dioxide trapping efficiency when presented with pure carbon dioxide. In laboratory culture experiments with simulated flue gas, the green alga Monoraphidium minutum could tolerate 200 ppm sulfur dioxide and 150 ppm nitric oxide. Some clues to fate of NO are found in the analyses of nitrite and nitrate concentrations in the culture medium during sparging with flue gas. Nitrite concentration in the culture media of flue gas treated cultures is much higher than in control cultures which did not receive sulfur dioxide and nitric oxide. This suggests that some of the NO may be dissolving and could be available as an N-source for the microalgae. Similarly, nitrate utilization is less in flue gas treated cultures, but cell growth is unaffected, again suggesting that N-metabolism is tied to flue gas tolerance. Overall, these results demonstrate that this type of simulated flue gas is tolerated very well by microalgae, and is an excellent substrate for their growth. Culture pH remains quite stable during these experiments indicating that sulfur dioxide is not likely to be a problem under this sparging regime, at least as far as its effect on culture pH.