Microalgae grow fast and take up CO2 through photosynthesis so they may have a role in CO2 capture. Microalgae have some other advantages which make them suitable for capturing CO2 from coal combustion flue gas:
• They do not require high purity CO2 gas which reduces the requirement to separate CO2 from flue gas
• Some combustion products such as NOx or SOx can be used as nutrients for microalgae which reduces the need for flue gas scrubbing systems
• Microalgae could yield high value commercial products
• The process is envisioned as a renewable cycle with minimal negative impacts on the environment.
Using microalgae to capture CO2 is a complex process, especially in flue gas environments. There are many factors to consider. Physico-chemical ones include CO2 concentration, presence of pollutants in the flue gas, the initial inoculation density, culture temperature, light, nutrients and pH; hydrodynamic parameters include 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 so 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. Open culture systems are normally cheaper to build and operate, more durable and have a large production capacity compared with large closed reactors.
However, open ponds are more susceptible to weather conditions, and do not allow the control of the culture medium temperature, water evaporation and light. Potential contamination is also a serious threat to the operational success of outdoor open ponds or raceways. Most importantly, they require an extensive land area and consume large amounts of water.
In contrast, closed system photobioreactors have more operational stability and condition control. But the capital and operating costs are higher and are barriers to the mass cultivation of microalgae.
Technologies are available to harvest, process and produce valuable products from microalgae. Most of the existing technologies are adapted from those already in use in the food, biopharmaceutical and wastewater treatment sectors and have not been developed specifically for algae production. As a result they are inefficient and require a large amount of energy. The economics of carbon fixation by algae could be improved by work in this area.
Selecting energy efficient harvesting and processing methods and high value strains to produce commercially sound applications is also a key to promoting capture of CO2 by microalgae. Flue gas transport is another issue. Keeping algae cultivation systems close to the CO2 source is one solution to avoid the cost of building long pipelines. But, microalgae cultivation requires a large land area. For new power plants to use microalgae bio-fixation as a CCS approach, a site with land available for large scale cultivation would be needed. This land requirement could be a problem for existing power plant.
Another potential advantage of the bio-CCS approach is the combination of CO2 fixation, biomass production and wastewater treatment. The nitrogen and phosphorous compounds in wastewater can be used by some algae strains.
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 utilisation efforts to the marketplace as a viable alternative to conventional CCS. They need a large, 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 of 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.
The report Microalgae removal of CO2 from flue gas CCC/250 by Xing Zhang, 95 pp May 2015 is available for download from the IEA Clean Coal Centre Bookshop http://bookshop.iea-coal.org.uk/site/uk/clean-coal-technology-research-reports. Residents of member countries and employees of sponsoring organisations can download the report at no charge after a one-off registration.