In April 2013 at the site of the, a green coloured house captured people’s attention. The outer layer of the house was made of algae panels. Flue gas and water were pumped into the panels for algae to grow. Apart from serving the conventional purpose of insulating the building from sound, heat and cold, the algae facades produced heat and biomass to supply the building with energy from renewable sources. In the meantime, it removed CO2, NOx and SOx from the flue gas. This algae house is still a showcase of the concept of biological post-combustion CO2 capture. Using microalgae to capture CO2 is a complex process, especially in flue gas environments. There are many factors to consider, such as CO2 concentration, the 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 with each other.
WHICH SPECIES? The choice of microalgae species is also important as it directly influences the photosynthesis efficiency, and hence, the performance of carbon fixation and biomass production. The desirable 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. Also, the economics of CO2 capture can be significantly improved if the algae products can be sold.
WHERE? Microalgae cultivation can be carried out in an open pond or closed photobioreactor systems. Open culture systems are usually cheaper to build and operate, more durable and have a large production capacity compared to largely closed reactors. However, they 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. However, the high capital and operating costs of photobioreactors are the barriers impeding the mass cultivation of microalgae. The key to promoting the use of microalgae to capture CO2 is to make the photobioreactors cheaper.
THE LIMITS Technologies are available to harvest, process and produce valuable products from microalgae. But 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 significant amount of energy. The economics of carbon fixation by algae could be improved by work in this area.
CARBON AND CAPTURE UTILISATION Co-firing dried microalgae with coal to produce electricity is the easiest and most obvious way to utilise microalgae. However, since microalgae contain lipids (7–23%), carbohydrates (5–23%), proteins (6-52%) and some fat, depending on the species, these constituents can be converted into several commercial applications, such as human food, animal feed, cosmetics, medical drugs, fertilisers, bio-molecules for specific applications and biofuel. For the power generation industry, these algae applications are an extra bonus after capturing CO2 from coal combustion because of the generated revenue. Therefore, although it has the same drawbacks as conventional carbon capture and storage methods, namely large energy requirement and equipment cost, CO2 mitigation by microalgae can be classified as carbon capture and utilisation due to the production of value-added biomass. Microalgae capture and convert CO2 into useful products. Thus, CO2 becomes a feedstock instead of a waste product.
POTENTIAL It is clear that using microalgae to capture of CO2 is technically feasible and has economic potential. Selecting efficient energy harvesting and processing methods and high-value strains to produce commercially sound applications is 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 requirement could be a problem for existing power plants. At the moment, 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 the CO2 fixation. Power companies will only be willing to invest significant 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. 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 paramount for algae companies and power companies to form a win-win partnership to share the costs and profits.