Micro-algae are a group of unicellular or simple multicellular fast growing photosynthetic microorganisms that can conserve CO2 efficiently from different sources, including the atmosphere, industrial exhaust gases, and soluble carbonate salts. Micro-algae act as a major system for converting atmospheric CO2 into lipids under sunlight and increase the output of algal oil. The enzyme acetyl Co-A carboxylase (ACCase) from micro-algae catalyses the key metabolic step in the synthesis of oil in algae.
See also: 'Biofuels from algae'.
About 3,000 species out of 200,000 species were found to be useful for sequestration of CO2 and the production of biodiesel (Keffar and Kleinheinz, 2002). Micro-algae are a promising alternative to CO2 mitigation by CO2 fixation, biofuel production, and wastewater treatments. CO2 fixation by photoautotrophic algal cultures has the potential to diminish the release of CO2 into the atmosphere, thereby helping to alleviate the trend toward global warming (figure 1). Biofuel is derived from microbes that can live on land unfit for crops and generate nearly engine-ready chemicals which are considered to be third generation biofuels (New Scientist, 2011).
Micro-algae, when fed with CO2 and sunlight, produced large amounts of lipids and hence increase the output of algal oil. The enzyme Acetyl CoA Carboxylase (ACCase) from micro-algae helps to catalyse and transform CO2 in the synthesis of oils in algae.
Technological developments, including advances in photo bioreactor design, micro-algal biomass harvesting, drying, and other downstream processing technologies are important areas that may lead to enhanced cost-effectiveness and therefore, effective commercial implementation of the biofuel using a micro-algae strategy.
Micro-algae can fix carbon dioxide from different sources, which can be categorised as:
- CO2 from the atmosphere.
- CO2 from industrial exhaust gases (e.g., flue gas and flaring gas).
- Fixed CO2 in the form of soluble carbonates (e.g., NaHCO3 and Na2CO3).
- Can be grown in closed systems, which could result in savings of precious freshwater resources.
The systems for using micro-algae for CO2 sequestration involve the following sub systems:
1) The open pond system
The size of open pond micro-algae production systems typically ranges from 0.22-0.4ha (Pedroni et al., 2001). An even larger (900ha) single algae production system has been reported from Mexico City (Becker, 1994). Similarly, the Arizona Department of Environmental Quality reported an algal growing pond of 1,406ha in Florida (Arizona Department of Environmental Quality, 1995). Advantages for utilising the open pond system are low initial and operational costs. Disadvantages of open pond system are the enormous size of the area required, which is not affordable in many regions, and a high water requirement.
2) The closed photo-bioreactor system
Photo-bioreactors provide advantages such as large surface/volume ratios, a barrier to minimise contamination, a capacity to achieve a high density of biomass, a high biomass productivity, and therefore, high CO2 fixation rate (Rosello et al., 2007). The tubular photo-bioreactor is one of the most popular configurations of photo –bioreactors used in algal carbon sequestration process (Travieso et al., 2001).
- The photo-bioreactor system has a higher potential productivity due to better environmental control and harvesting efficiency.
- Even though the open pond systems seem to be favored for commercial cultivation of micro-algae at present due to their low capital costs, closed systems offer better control over contamination, mass transfer, and other cultivation conditions.
- Closed photo-bioreactors require less fresh water than open ponds. However, cooling systems that utilize water may be needed to cool the reactors under excessively warm conditions, although poorer quality water may be utilised for the cooling.
- Photo-bioreactors are highly uneconomic due to their prohibitive cost.
- Photo-bioreactors can be used only for micro algal strains that are easily harvested.
3) Environmentally controlled system
Another strategy explored for CO2 sequestration use by algae is to build moderate environmentally controlled systems, such as greenhouses. Growers can control the environment inside greenhouses while construction costs are not as high as a photo-bioreactor with a solar collector system.
- Micro-algal CO2 bio-mitigation can be made more economic, cost-effective, and environmentally sustainable, especially when it is combined with other processes such as wastewater treatment. The utilisation of wastewater for micro-algae cultivation will bring about remarkable advantages including the following:
- Micro-algae have been shown to be efficient in nitrogen and phosphorus removal (Mallick, 2002), as well as in metal ion depletion, and combination of micro-algae cultivation with wastewater treatment will significantly enhance the environmental benefit of this strategy.
- It will lead to savings by minimising the use of chemicals such as sodium nitrate, potassium and phosphorus as exogenous nutrients.
- Micro-algae have much higher growth rates and CO2 fixation abilities compared to conventional forestry, agricultural, and aquatic plants (Li et al., 2008).
- Some micro-algae species, such as Chlorella, Spirulina and Dunaliella have commercial values. It is expected that commercial profit from biomass production will offset overall operational costs for CO2 sequestration.
- Species such as Chlorella can grow under 20% CO2 conditions, and therefore, they can use industrial exhaust gases for a CO2 source, and they can be used as a health food (Becker, 1994).
- Some micro-algae (eg. Dunaliella) use CO2 to produce secondary metabolites such as β-carotene, fertilisers, and biofuels as byproducts of economic importance. These products are used as food, medicine and cosmetic products. They also produce cost-effective biofuel (Graham and Wilcox, 2000).
- Micro-algae are also considered as multifunctional systems which are used as waste treatment, especially for the removal of nitrogen and phosphorus from effluents (Mallick, 2002) and in aqua culture farms, as well as being an environmental friendly technology.
- The high growth rate of micro-algae makes it possible to satisfy the massive demand for biofuels, using limited land resources without causing potential biomass deficit.
- Micro-algal cultivation in closed systems consumes less water than land crops.
- The tolerance of micro-algae to high CO2 content in gas streams allows high-efficiency CO2 mitigation (figure 5).
- Nitrous oxide release could be minimised when micro-algae are used for biofuel production.
- Micro-algal farming could be potentially more cost-effective than conventional farming.
- Micro-algal farming can be coupled with flue gas CO2 mitigation and wastewater treatment.
- A low biomass concentration in the micro-algal culture must be maintained in order not to limit light penetration which in combination with the small size of algal cells makes the harvest of algal biomasses relatively costly.
- The cost of production is very high.
Obtaining biofuels from micro-algae is a research topic at several locations around the world. However, commercial production does not yet exist. According to Wijffels and Barbosa (2010), current worldwide micro-algal manufacturing infrastructure can produce only about 5,000 tonnes of dry algal biomass per year, and that is devoted to extraction of high value products, such as carotenoids and omega-3 fatty acids for food and feed ingredients.
The main barrier is the enormous cost of production, as well as practical aspects, such as harvesting and drying. Wijffels and Barbosa (2010) estimate an area the size of Portugal would be needed to supply the transport fuel needs of Europe from micro-algae, so the scale of production would have to increase by three orders of magnitude. They also state that a concomitant decrease in the cost of production by a factor of 10 is needed.
As per Schenk et al. (2008) and Benemann and Oswald (1996) the cost of algal oil production comes in the range of US$52–$91 per barrel. This estimate was based on 400 hectares of open ponds, using either pure CO2 or flue gas from a coal-fired power station and productivity assumptions of 30-60 g m−2 day−1 with 50% algal lipid yield. Such high yields are theoretically possible but to date have not been demonstrated. Another analysis (Huntley and Redalje, 2006) estimated algae oil production costs to be US$84 bbl. This scenario was based on the infrastructure cost assumptions utilising a hybrid system with an aerial productivity of 70.4 g m−2 day−1and 35% algal lipid yield.
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