Cultivation of algal biomass could provide a method of carbon mitiga­tion through CO2 uptake from flue gases during photosynthesis. Provid­ing algae can utilise industrial gases, there is the potential to remove CO2, which would otherwise be emitted. The mitigation of CO2 from flue gas using algae would be ideal in an industrial scenario, as targets for reducing greenhouse gas emissions are becoming tighter [44]. The improvement of biomass yields by introducing a concentrated source of CO2 has been reported [45, 46], however, there are many barriers yet to overcome; for example concentration of CO2 in flue gas may be too high for many strains of algae resulting in toxicity, and/or the presence of other toxins in the gas may adversely affect productivity, and/or gas transport cost to algal biomass growth reactors or ponds may be unviable. Nevertheless, as men­tioned, there is certainly potential for flue gases to play a part in an algal biomass cultivation system.

The atmosphere provides a CO2 concentration of0.038% for the growth of algae; theoretically, with a higher concentration available, higher pro­ductivity is possible [47]. Early studies found Chlorella sp. to be highly suitable for cultivation in flue gases due to its capacity to be grown with the injection of gas containing a CO2 concentration of 15% [48], a concen­tration similar to that of most flue gases [46]. Experimentation conducted within the US aquatic species programme [15] using flue gases as a source of CO2 indicated that local strains of algae dominated with a high CO2 use efficiency. Single algal biomass productivity rates as high as 50 g/m2/ day were recorded, although attempts to achieve consistently high produc­tivity rates failed during a long-term experiment for one year, provably due to low ambient temperatures [15]. In 2002, research was conducted

by the National Renewable Energy Laboratory (NREL) and the US De­partment of Agriculture investigating uptake of CO2 from synthetic and flue gas sources and its commercial and environmental viability. The technical feasibility and economic viability of integrating a micro-algal cultivation system with a coal fired power plant was investigated [49], using a bench scale system as a test rig. An artificial flue gas (12% CO2; 5.5% O2; 423 ppm SO2; 124 ppm NOx) based on the composition of a North Dakota power sta­tion boiler was produced and sparged into a bio-reactor tank. Two strains of algae were cultivated, Monoraphidium and Nannochloropsis, both of which grew successfully under the administered conditions. It was reported that growth rates of the microalgae varied between 15 to 25 g/m2/day and con­tained 41% protein, 26% lipid and 33% carbohydrate [49].

Research using real flue gases for CO2 uptake and cultivation of algal biomass has also been conducted [45, 46, 50, 51]. For example, Chlorella sp. was cultivated using a photobioreactor system approach and the pro­ductivity of Chlorella sp. was investigated in presence of a flue gas (6-8% CO2) from a natural gas boiler and in presence of a control gas, which resulted in higher productivity in the flue than in the control gas, of 22.8 ± 5.3 g/m2/day [52]. As a result it was suggested that 50% of the flue gas could be decarbonised using that system [51]. Similar studies conducted with Chlorella vulgaris using a photobioreactor system approach and flue gas from a municipal waste incinerator indicated that this strain was toler­ant to a concentration of 11% (v/v) CO2 as well as to the flue gas, with a higher biomass productivity in the flue [46]. Both studies suggest that the presence of potential contaminants in the flue had little adverse impact upon the algae. Examples of various strains of algae cultivated with the addition of CO2 with their productivity rates are summarized in Table 2. Existing research indicates that an improved growth of algal biomass has been obtained using artificial and flue gases with CO2 concentration up to approximately 12% (Table 2). Above this concentration it appears that productivity is reduced, most likely due to acidity caused by the high CO2 levels. It is suggested that, although most strains would benefit from an increased concentration of CO2, testing is required to identify optimal CO2 concentrations as this appears to vary between strains. Similarly only a limited number of flue gas sources have been investigated; if the concept is likely to be taken up across many different industries, a variety of flue gases will need to be tested.

In summary, research to date suggests mitigation of CO2 using algae cultivation is promising providing the gases are at a low concentration and contain low levels of contamination and operational conditions (e. g., pH, temperature, light) are controlled. Additionally the cost and energy input of transporting the gases to the ponds or bioreactors must be balanced by the benefits that the extra CO2 and the carbon mitigation provide. In a study investigating the potential for using power plant flue gas [54], it was estimated that an electricity consumption of 15.1 kWh/day was necessary for the direct injection of CO2 to one hectare of cultivation pond. The en­ergy benefits as a result of the injection would therefore need to be at least

15.1 kWh/d/ha and the CO2 savings greater than what would be emitted by conducting injection.