Photobioreactor for CO₂ Sequestration: Possibilities and Challenges

There are multiple parameters to be taken care of for algal mediated CO₂ sequestration. The very basic and primary step is strain selection. Certain desirable qualities include tolerance to high CO₂ concentration, high biomass productivity, tolerance to variations in temperature and most importantly high CO₂ fixation rates. Most commonly used algal species are depicted in Table 1.

Strain selection is directly linked with mode of cultivation. The conditions of cultivation of algae have a peculiar effect on end result. The photoautotrophic cultivation mode utilizes light for energy requirement and inorganic carbon as carbon source. In this mode, there are large variations in lipid content which can be tilted towards user’s advantage by increasing the lipid accumulation through nitrogen limiting condition [5,9]. Totally opposite mode of cultivation is heterotrophic condition where algae utilize organics for both energy as well as nutrient requirement. Since it does not require light, hence it is under the dark conditions [10,11]. Another mode of cultivation is the mixotrophic growth where light is used an energy source and depending upon the availability, both organics as well as inorganics are used as nutrient source. This is particularly required in cases where algal cultures are cultivated in wastewater with CO₂ injection. In such cases, algae utilizes light for energy, CO₂ as carbon source while simultaneously utilizing organics for treatment of wastewater hereby making the process more economically viable [7,12,13].
 

Once the strain selection is made, well designed photobioreactor becomes need of the hour. Cultivation could be done in open as well as closed system. Open systems, like unstirred ponds, circular ponds, raceways and modified raceways offer ease of operation and are more economical than closed system [22]. Major constraints offered by open systems include poor light utilization efficiency, huge evaporative losses and biological contamination. For high rate of CO₂ sequestration, open system are not viable as gas mixing becomes a challenge due to low depth of ponds [23]. When the efficacy of algal mediated CO₂ sequestration was tested in outdoor open photobioreactor, the results indicated that flue gas decarbonisation decreased with increasing flue gas injection rate into culture [17].

For high rate of CO₂ sequestration, mainly closed type photobioreactors are employed. The various design parameters for closed type photobioreactors include type of reactors, height to diameter ratio, mode of delivery, minimizations in temperature variations, optimum intensity of light, maximum utilization of light, proper mixing, optimum light - dark cycles, minimization of CO₂ losses. As depicted in Table 1, multiple designs of photobioreactors present multiple opportunities and challenges. These include mainly tubular type photobioreactors (horizontal and vertical), stirred tank photobioreactors and flat panel photobioreactors. Tubular photobioreactors are fairly easy to operate with decent biomass productivities due to high illumination surface area [22]. This system incorporates gas sparging as a means to provide overall mixing and high gas transfer efficiency [24]. Other variants include bubble column photobioreactor and airlift photobioreactor [25]. In a three stage tubular reactor, though the CO₂ fixation rate was high yet the problems associated with flocculating Spirulina sp. highlighted the importance of strain selection [14]. In the study concerning vertical tank reactors, it was noted that the algal sequestration of CO₂ could be increased by multiplicity of reactors hereby presenting an insight with regard to scale up [16]. In case of membrane photobioreactor, gas (CO₂) exchange efficiency improved from the introduction of membrane module hereby resulting in minimization of losses [15]. While in membrane carbonation photobioreactor the bubble less gas transfer through membranes allows precise control of CO₂ delivery hereby minimizing the loss of CO₂ to atmosphere [18]. There is also need to highlight the maximized fixation of CO₂ by optimizing the light intensity for minimization of CO₂ losses [20].

Light intensity can also be increased by better designs (internally illuminated) where light utilization rate is significantly higher as compared to bubble column and airlift reactors [21]. Studies have exhibited equal importance of dark cycles for algal growth. The dark cycle allows for re-oxidation of electron transporters of photosynthetic apparatus for sustained productivities [26]. Even the importance of optimum concentration where one has to remove the excess biomass continuously in order to provide maximum spread area to volume ratios, can’t be ignored [27].
 

Keeping in mind the process economics, significant economical addition to the entire process can be done through harvesting of algal biomass and then use of algal biomass for bioenergy generation [7]. Depending upon the scale, size and density of algal culture, various harvesting techniques like centrifugation, flocculation, gravity sedimentation, filtration, electrophoresis etc. can be employed. Centrifugation can result in high harvesting efficiency but the process is costly and energy intensive [28]. Flocculation is a process where particles form heavy aggregates which make their settling easily [29]. This could be done via increase in pH (autoflocculation) through base (NaOH) addition or through chemical flocculants like ferric chloride (FeCl₃) or aluminium sulphate Al₂(SO₄)₃ [24]. Success of gravity sedimentation highly depends upon size and density of algal cultures [30]. Efficiency of gravity sedimentation can be increased using flocculation [29]. Filtration involves passing of culture through screen of particular pore size. Major associated problem with filtration is high concentration of algal biomass often results in blocking of filtering material [31]. Other techniques include electrophoresis where electric current induced electrostatic field forces the movement of charged algal cells out of solution [32].
 

The ENN group from China has developed a technology where a microalga is used to fix CO₂ released from coal production and subsequently biofuel is produced. The pilot plant consists of equipment required for microalgae cultivation with CO₂ absorption, oil extraction and bio-diesel production. This pilot system can absorb 110 tonnes of CO₂ and produce 20 and 5 tonnes of bio-diesel and proteins per year, respectively. Based on this pilot plant, a demonstration project has been initiated by ENN group in Dalate (Mongolia) in 2010 where microalgae will be utilized to absorb CO₂ emitted from flue gas of coal-derived methanol and coal derived dimethylether production equipment and will produce biodiesel and feed.

Another group from Sweden, has set up a pilot plant in eastern Germany where algae is used to absorb greenhouse gas emissions from a coal-fired power plant. The gas emitted from the brown-coal-fired plant is pumped through a broth into the plastic tanks where algae is cultivated. The resultant algal biomass is used to produce biodiesel, to feed biogas power plants and as a nutrient supplement for fishes.