Micro algae have the ability to fix CO2 using solar energy with an efficiency 10 times greater than that of terrestrial plants (Usui and Ikenouchi, 1997), with numerous additional technological advantages.
Previous studies have shown that there are strains of green algae capable of growing rapidly under high CO2 concentrations (≤50%) (Hanagata et al., 1992; Sakai et al., 1995; Satoh et al., 2002; Sung et al., 1999) and also in the presence of SOx and NOx which are common contaminants in exhaust gas from fossil fuel combustion (Brown, 1996; Maeda et al., 1995; Yoshihara et al., 1996; Zeiler et al., 1995). Exposure to high CO2 concentrations have shown to increase the production of high value biomolecules (Miyasaka et al., 1998). Considerable effort has been invested in constructing bioreactors for the purpose of efficient CO2 fixation into algal biomass (Cheng et al., 2006; Keffer and Kleinheinz, 2002; Ono and Cuello, 2004; Pulz, 2001; Sato et al., 2006; Usui and Ikenouchi, 1997).
Large scale bioreactors in use today can be divided into open or closed systems, and it seems evident that a closed system will be an advantage for the purpose of maximum fixation of CO2 and also for the purpose of obtaining non-contaminated biomass for extracting biomolecules. A CO2 fixation efficiency of 260 mg/ l/h_1 has been obtained in lab scale systems (10 l) (Cheng et al., 2006), and if this rate can be maintained in a large scale bioreactor, it would equal CO2 capture of 26 kg/ h in a 100,000 l reactor or 114 t /year assuming 12 h-days. Calculations of economics for commercial production of algal biomass showed that the cost of CO2 purchased from the market constitutes 40% of the raw material expenses of the production (Molina Grima et al., 2003). However, a recent feasibility model (Ono and Cuello, 2006) concluded that CO2 mitigation by microalgae will only be economically feasible when exploiting the biomass by for example extracting high value by-products. An integrated system where the algal biomass is used for creating revenues in the form of health food/aquaculture and in addition H2 production followed by biomolecule extraction, fertilization, biofuel and combustion, could be a sustainable process.
Kajiwara et al. [1997] found that Synechococcus achieved a maximum CO2 uptake rate of 0.025 g/l/h or 0.6 g/l/day at a cell mass concentration of 0.286 g/l. If scaling up were plausible, this would equate to a bioreactor of size 4000 m3 with an average fixation rate of 1 tonne CO2/h from emission sources.
Hirata et al. [1996] used Chlorella sp. UK001, a type of micro-algae, to produce a photosynthetic reaction. The mean rate of CO2 fixation was 0.0318 gCO2/l/day. The efficiency of conversion of energy to biomass was estimated as 4.3%. This species of micro-algae can grow in an atmosphere containing 3–40% CO2 with pH between 5.5 and 6.0 and temperature of around 30 C. Maeda et al. [1997] found that Chlorella sp. T-1 was highly resistance to temperature and high concentrations of CO2. The strain was tested at 35 C and 15% CO2, making this particular strain ideal for biological fixation of CO2 exhausted from coal fired plants. The cultivation of this strain was satisfactory even when actual flue gas was used. This demonstrated the strain’s resistance not only to CO2, SOX and NOX , but also to chlorine, fluorine and dust.
Murakami et al.(1997) using Synechocystis aquatilis in a 5 l bioreactor and optimised conditions, reached a maximum CO2 fixation rate of 1.5 gCO2/ l/day. Further experimentation using Botryococcus braunii, a growth rate of around 0.5 g/l/day was achieved. This particular algal biomass contained more than 15% of its dry weight as hydrocarbon, and its CO2 fixation rate exceeded 1 g/ l/day. The optimal growth temperature range for this algae was 25–30 C. Assuming that the carbon uptake rate of 1.5 g/l/day [Murakami et al.(1997)] for the particular micro-organism Synechocystis aquatilis could be sustained for the natural light cycle over a 24 h period, for a 4000 m3 pond, up to 2.2 ktonne CO2/pond/year could be sequestered from the environment. It has been illustrated that a 4000 m3 pond under natural daily light exposure cycles could sequester up to 2.2 ktonne of CO2 per year.
To maximize both the economical and energetic efficiency of the process, it will be necessary to use species of algae which have high CO2 fixing and solar energy conversion efficiency
Work carried out by PHYCO SPECTRUM: Micro algal species which can poisitively respond to increased CO2 levels have been identified and isolated. Laboratory trials have established their effeciency to assimilate CO2 at a faster rate. These micro algae can be employed very effectively in the mitigation of CO2 from flu gas from industries. For further detials and consultancy contact Email:
phycospectrum@gmail.com
