sing microalgae to capture CO2 is a complex process, especially in flue gas environments, reports an editorial by IEA Clean Coal Centre in worldcoal.com. There are many factors to consider, such as CO2 concentration, presence of pollutants in the flue gas, the initial inoculation density, culture temperature, light, nutrients and pH, as well as hydrodynamic parameters including flow, mixing and mass transfer. The growth of microalgae and its tolerance to the environment depends on all the process factors and how they interact.
The choice of microalgae species is also important as it directly influences the photosynthesis efficiency and subsequently the rate of carbon fixation and biomass production. The best microalgae species for capturing CO2 need to have a fast growth rate, a high rate of photosynthesis, strong tolerance/adaptability to the trace constituents of flue gas, high temperature tolerance, the possibility to produce high value products and be easy to harvest and process. The economics of CO2 capture can be significantly improved if the algae products can be sold.
Microalgae cultivation can be carried out in open pond or closed photobioreactor systems. Selecting energy efficient harvesting and processing methods and high value strains to produce commercially sound applications is also key to promoting capture of CO2 by microalgae.
In her latest report for the IEA Clean Coal Centre, “Microalgae removal of CO2 from flue gas,” Xing Zhang found that the CO2 fixation rate of microalgae tends to be too low to compete with conventional CCS methods. Using flue gas to culture algae is more applicable to the production of high value products than CO2 fixation.
Power companies will only be willing to invest large amounts of capital, land and water if the microalgae products can be sold at a good price. Algae companies are almost ready to bring their bio-carbon capture and utilization efforts to the marketplace as a viable alternative to conventional carbon capture and storage (CCS). They need a large and constant amount of CO2 for the technology to work. However, those strains, which can thrive under flue gas conditions, do not often have a high commercial value.
If algae companies have to pay the power companies to reuse the flue gas, they may not have the motivation to produce low value algae biomass just for the purpose of endorsing CCS. Therefore, it is very important for algae companies and power companies to form a win-win partnership to share the costs and profits.
It is clear that using microalgae to capture CO2 is technically feasible and has economic potential. But before cheap, efficient photobioreactors become available, algal capture of CO2 is better viewed as a means of providing high value end products rather than as a direct competitor to conventional CCS technology.
—Edited by Harleigh Hobbs