Qualitative assessment of capturing CO2 from industrial flue gases by algal cultivation

Technical advances and the increased urgency to reduce greenhouse gas emissions have lately revived the research into biofuels from microalgae. Research has shown that microalgae can be cultivated using flue gases. Therefore, integrating algal cultivation with CO2-containing flue gases or vent gases from industrial sources provides an interesting opportunity for capturing CO2 and biochemically converting it into bio-based products such as fuels. In this work, technical solutions for separating CO2 from CO2-containing flue gases for feeding an algal cultivation were qualitatively evaluated, with focus on processes that can maximize the amount of CO2 separated and converted to organic carbon. In Finland, experimental work using combined heat & power flue gas was performed to test the tolerance of various species towards potentially toxic substances in the flue gas, such as nitrogen oxides and sulphur oxides. In India, microalgae suitable for cultivation in vent gas from a natural gas processing facility were screened, after which cultivation in a test unit consisting of a CO2 absorption column and raceway pond were performed. Sufficient CO2 supply to an algal cultivation is an important engineering issue, as overfeeding CO2 can turn the water too acidic for the algae cultivation, while underfeeding CO2 cause carbon starvation of the cultivation and/or increase the pH to levels inhibiting algal growth. Pumping of excessive amounts of flue gas through the cultivation reactors or ponds requires energy and increases cost. Pure CO2, as used in current commercial algae production, is expensive, since separation of pure CO2 from a flue gas stream requires additional investments and energy. CO2 supply systems are designed and operated so that the pH can be kept at levels suitable for maintaining the cultures while minimizing loss of CO2 to the atmosphere. Still, the CO2 gas loss to the atmosphere from the algal cultivation is relatively high. As algae can thrive using CO2 from desulphurized flue gases injected into the cultivation water, there is no need for costly CO2 separation processes, as long as the algal cultivation is built next to a suitable industrial CO2 source. The most promising systems identified for CO2 capture for algae cultivation are processes that use separate bubbling carbonation columns, both for open ponds and closed photobioreactors. This makes the design of the photobioreactors simpler and lowers significantly the energy requirements for pumping flue gas, as CO2 readily dissolves in the feed water stream. The spent cultivation water is recycled back to the bubbling columns, where flue gases are bubbled through the water, absorbing gaseous CO2. Using separate bubbling columns for open ponds enables a higher CO2 concentration in the ponds than what can be achieved by direct injection, and reduces the risk for release of gaseous harmful flue gas components into the area surrounding the ponds. While the capacity of pure water to dissolve CO2 is poor, the addition of alkaline salts can significantly improve the CO2 absorption capacity, without turning the water too acidic for the algal cultivation. According to previous studies, algal growth can be reduced due to the presence of potentially toxic compounds in industrial off-gases, especially hydrogen sulphide, sulphur oxides and nitrogen oxides, as well as decrease in pH caused by an oversupply of CO2. Therefore, experimental work with microalgae cultivation using both flue gas from a coal-fired combined heat and power (CHP) plant and vent gas from a natural gas processing facility was performed. Growth of three microalgae species 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. The species included two green algae, one diatom, and one cyanobacterium. Gas supply was adjusted according to the carbon uptake capacity of the microalgae. The growth was observed using fluorescence and optical density measurements as proxies for biomass, and the final biomass concentrations were determined by weighing filtered samples. No significant statistical differences (p>0.05) 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 natural gas processing facility were screened by employing a 20 L photobioreactor (PBR). CO2 tolerance of various species was studied by sparging CO2 and monitoring pH. Mixtures of species were also tested. Based on these experiments, a certain mixture of microalgae exhibited rapid growth and better tolerance in terms of time taken to reach pH 7. Larger-scale cultivation of the mixture was tested 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. These results show that cultivation of microalgae with industrial off-gases as carbon source is possible, but also that species selection has to be considered as an essential part of the production process optimization. Furthermore, the results indicate that algal carbon capture might reduce the need for desulphurization or denitrification, as there were indications that microalgae were able to utilize flue gas nitrogen and sulphur as nutrients. Whilst the results are encouraging the long-term effects need to be verified by conducting experiments at a larger scale with continuous cultivation. Also, the economic feasibility of concept needs to be investigated. This work was carried out in the Carbon Capture and Storage Program (CCSP) research program coordinated by CLEEN Ltd. with additional funding from Finnish Funding Agency for Technology and Innovation, Tekes.