Closed Hybrid System

Other system designs for algae production are possible. The Japanese, French, and German governments have invested significant R&D dollars on novel closed photobioreactor designs for algae production. The main advantage of such closed systems is that they are not as subject to contamination. When designing a photobioreactor, design parameters such as reactor dimension, flowrate, light requirements, culture condition, algae species,

reproducibility, and economic value need to be taken into consideration. Depending on the reactor dimensions, site location, and local climate, these parameters can determine the type of cultivation system needed (open versus closed). Reactor design should have good mixing properties, efficiency, and reproducibility and be easy to maintain and sterilize. An efficient photobioreactor not only improves productivity but also is used to cultivate multiple strains of algae. The performance of a photobioreactor is measured by volumetric productivity, areal productivity, and productivity per unit of illuminated surface (Riesing, 2006). Volumetric productivity is a function of biomass concentration per unit volume of bioreactor per unit of time. Areal productivity is defined as biomass concentration per unit of occupied land per unit of time. Productivity per unit of illuminated surface is measured as biomass concentration per area per unit of time. Closed photobioreactors support up to fivefold higher productivity with respect to reactor volume and consequently have a smaller “footprint” on a yield basis.

Besides saving water, energy, and chemicals, closed photobioreactors have many other advantages that are increasingly making them the reactor of choice for biofuel production, as their costs are lower (Schenk et al., 2008). Closed photobioreactors permit essentially

single-species culture of microalgae for prolonged periods. Most closed photobioreactors are designed as tubular reactors, plate reactors, or bubble column photobioreactors (Pulz, 2001).

Other less common designs like semihollow spheres have been reported to run successfully (Sato et al., 2006). Closed photobioreactors have been employed to overcome the

contamination and evaporation problems encountered in open ponds (Molina Grima et al., 1999). These systems are made of transparent materials and are generally placed outdoors for illumination by natural light. The cultivation vessels have a large surface-area to-volume ratio.

The main problems in the large-scale cultivation of microalgae outdoors in open ponds are low productivity and contamination. To overcome these problems, a closed system consisting of polyethylenes sleeves was developed. The closed system was found to be superior to open ponds with respect to growth and production in a number of microalgae. In both closed and open systems, growth and production under continuous operation were higher than in batch

cultivation (Cohen et al., 1991). The preferred alternative is closed photobioreactors, where the algae fluid remains in a closed environment to enable accelerated growth and better control over environmental conditions. These glass or plastic enclosures, often operated under modest pressure, can be mounted in a variety of horizontal or vertical configurations and can take many different shapes and sizes. Rigid frameworks or structures are usually used to support the photobioreactor enclosures.

In hybrid systems, both open ponds as well as closed photobioreactor system are used in combination to get better results. Open ponds are a very proficient and lucrative method of cultivating algae, but they become contaminated with superfluous species very quickly. A combination of both systems is probably the most logical choice for cost-effective cultivation of high yielding strains for biofuels. Open ponds are inoculated with a desired strain that had invariably been cultivated in a photobioreactor, whether it is as simple as a plastic bag or a high-tech fiber-optic bioreactor. Importantly, the size of the inoculums needs to be large enough for the desired species to establish in the open system before an unwanted species.

Therefore, to minimize contamination issues, cleaning or flushing the ponds should be part of the aquaculture routine, and as such, open ponds can be considered as batch cultures (Schenk et al., 2008). Abundant light, which is necessary for photosynthesis, is the third requirement.

This is often accomplished by situating the facility in a geographic location with abundant, uninterrupted sunshine (Brown and Zeiler 1993). This is a favored approach when cultivating in open ponds. Photobioreactors are flexible systems that can be optimized according to the biological and physiological characteristics of the algal species being cultivated, allowing one to cultivate algal species that cannot be grown in open ponds. A great proportion of light does not impose directly on the culture surface but has to cross the transparent photobioreactor walls.

In spite of their advantages, it is not expected that photobioreactor have a significant impact in the near future on any product or process that can be attained in large outdoor raceway ponds. Photobioreactors suffer from several drawbacks that need to be considered and solved. Their main limitations include: difficulty in scaling up, the high cost of building, operating and of algal biomass cultivation, overheating, bio-fouling, oxygen accumulation, and cell damage by shear stress and deterioration of material used for the photo-stage.

The cost of biomass production in photobioreactors may be one order of magnitude higher than in ponds. Whereas in some cases, for some microalgae species and applications it may be low enough to be attractive for aquaculture use; in other cases, the higher cell concentration and the higher productivity achieved in photobioreactor may not balance for its higher capital

and operating costs.

1.7 Factors influencing the growth of algae

Numerous aspects influence the growth and lipid content of algae. The reaction driving the initial conversion of sunlight into stored energy is photosynthesis. Therefore, all of the components involved in photosynthesis contribute to growth. The major factors include lighting, mixing, CO2 enrichment, O2 removal, nutrient supply, temperature, and pH (Suh and Lee, 2003a; Richmond, 2004b; Carvalho et al., 2006; Grobbelaar, 2009). It is important to note that in each category the precise conditions for optimal growth depend on the strain of algae selected for cultivation.

1.7.1 Light supply

An optimal reactor enhances light intensity/ penetration, as well as the wavelength of light and the frequency of cellular exposure to light.When selecting the light source, both the spectral quality and intensity must be considered. The spectral quality of light utilized by algae is defined by the absorption spectrum in the range of 400 to 700 nm for the chlorophyll and other photosynthetically active pigments, and the algal photosynthesis efficiency is a function of the spectral quality of the light source (Simmer et al., 1994; Suh and Lee, 2003a).

The level of light intensity is critical because at a certain level algae experience light saturation and dissipate the excess energy as heat (Mussgnug et al., 2007). Light saturation can be mitigated by the spatial dilution of light, which is the distribution of solar radiation on a greater photosynthetic surface are, and also reduces mutual shading of microalgal cells.

Thus, a design principle for photobioreactor designs is to maximize the surface area to volume ratio, which can be used for comparison between reactors.

Beyond the surface area and volume, the unique geometry of a reactor influences the light distribution. For example in a tubular reactor, the light gradient is primarily determined by the diameter of the tube and the biomass density in the medium (Janssen et al., 2003).

Optimal cell density is specific to each strain and needs to be maintained in order for light intensity and light penetration to remain at optimal levels (Richmond, 2004b). Light and dark cycles strongly influence the growth of algae. In both open ponds and outdoor closed reactors, natural light is subject to changes in time of day, weather, season, and geography (Pulz and Scheinbenbogen, 1998). Unfortunately, all reactors using natural light are subject to the

absence of light during nighttime. According to Chisti (2007), biomass losses might reach as high as 25% during the night, depending on the light intensity during the day, the temperature during the day, and the temperature at night. Janssen et al. (2003) stated that the length of the light/dark cycles experienced by algae influenced photosynthetic efficiency.

1.7.2 Mixing

The level of mixing in a reactor strongly contributes to the growth of algae. When environmental conditions do not limit growth rates, mixing is the most influential factor contributing to algae growth rates (Suh and Lee, 2003a). Mixing affects growth in two

primary ways: (1) improves productivity by increasing the frequency of cell exposure to light and dark volumes of the reactor and, (2) by increasing mass transfer between the nutrients and cells (Qiang and Richmond, 1996).

Mixing and lighting are closely related, as mixing is often responsible for inducing the light and dark cycles beneficial to algae growth. Similarly, mixing offers little benefit if lighting is poor and or culture in low density (Richmond, 2004b). Ugwu et al. (2005)

demonstrated that the installation of static mixers in tubular reactors succeeded in increasing light utilization and biomass yields when the reactor was scaled up by increasing the tube diameter.

1.7.3 Carbon dioxide consumption

In addition to light and water, CO2 is necessary for photosynthesis to occur. However, an excess of CO2 can also be detrimental to photosynthesis and cell growth. CO2 concentrations from 1 to 5% (by volume) often lead to maximum growth. Despite this, laboratories routinely aerate algal cultures with 5 – 15% CO2, or even pure CO2 (Suh and Lee, 2003a).

Flue gas is a desirable source of CO2 because it reduces greenhouse gas emissions as well as the cost of algal biofuel production. Flue gas from typical coal-fired power plants contain up to 13% CO2 (Chisti, 2007). Doucha et al. (2005) studied the performance of a closed reactor utilizing flue gas as a source of CO2 versus a reactor utilizing pure CO2.

Surprisingly, productivities and photosynthetic efficiencies were very similar under conditions of pure CO2 versus flue gas. Because CO2 concentration in flue gas was relatively low, the efficiency of CO2 mass transfer was lower for flue gas than it was for pure CO2.

1.7.4 Oxygen removal

A high presence of oxygen (O₂) around algae cells is undesirable. The combination of

intense sunlight and high oxygen concentration results in photooxidative damage to algal cells (Chisti, 2007).

Because of the constraint on the concentration of dissolved oxygen, tube length is limited in horizontal tubular reactors. This restriction makes it very difficult for tubular reactors to be scaled-up. In a tubular reactor designed by Molina et al. (2001), the algae culture regularly returned to an airlift zone where the accumulated oxygen from photosynthesis was stripped by air. A gas-liquid separator in the upper part of the airlift column prevented gas bubbles from recirculating into the horizontal loop of the airlift reactor. The time taken by the fluid to travel the length of the degasser must at least equal the time required by the oxygen bubbles to rise out.

1.7.5 Nutrient supply

In order to grow, algae require more than the reactants in the photosynthesis reaction.

Two major nutrients are nitrogen and phosphorus, which both play a role in controlling growth rates. Other essential nutrients are carbon, hydrogen, oxygen, sulfur, calcium, magnesium, sodium, potassium, and chlorine. Nutrients needed in minute quantities include iron, boron, manganese, copper, molybdenum, vanadium, cobalt, nickel, silicon, and selenium (Suh and Lee, 2003a).

1.7.6 Temperature

Temperatures experienced by algae grown outdoors can vary as much as the extreme outdoor temperatures characteristic to the geographic region of cultivation. Although algae may be able to grow at a variety of temperatures, optimal growth is limited to a narrow range specific to each strain. Seasonal and even daily fluctuations in temperature can interfere with algae production. Temperatures can reach as high as 30°C higher than ambient temperature in a closed photobioreactor without temperature control equipment (Suh and Lee, 2003a).

Evaporate cooling, water spray or shading techniques are employed frequently to inhibit temperatures of that magnitude. Whereas, a lower temperature appears to reduce the loss of biomass due to respiration during the night (Chisti, 2007).

1.7.7 pH

Each strain of algae also has a narrow optimal range of pH. The pH of the medium is linked to the concentration of CO2. Suh and Lee (2003a) mentioned that pH increases steadily in the medium as CO₂ is consumed during flow downstream in a reactor. The pH affects the liquid chemistry of polar compounds and the availability of nutrients such as iron, organic

acids, and even CO2 (Lee and Pirt, 1984). Because pH is so influential, Suh and Lee (2003a) stated that commercial pH controllers must be used in reactors to optimize growth.

1.8 CO2 Reduction by Microalgal Cultures

Generally, microalgae can typically capture CO2 from three different sources:

atmosphere, emission from power plants and industrial processes, and from soluble carbonate (Wang et al., 2008). CO2 capture from atmosphere is probably the most basic method to sink carbon, and relies on the mass transfer from the air to the microalgae in their aquatic growth environments during photosynthesis.

There are three main CO₂ mitigation strategies are normally used, (1) physical method, (2) chemical reaction-based approaches, and (3) the biological mitigation. Most of carbon capture and sequestration (CCS) discussions are about geological storage of CO₂ presently.

Whilst the oil and gas industry has successfully injected CO₂ into reservoirs, just before date this has mainly been for increased yield of fossil hydrocarbon reserves and not for long-term storage. This is proven safe but the biggest difficulty with this approach is the added cost of separation of CO2 from the emission streams (Packer, 2009). The chemical reaction-based CO2 mitigation approaches are energy-consuming, use costly processes, and have disposal problems because both the captured CO2 and the wasted absorbents need to be disposed of. In other hand, the biological CO2 mitigation has attracted much attention in the last years since it leads to the production of biomass energy in the process of CO2 fixation through

photosynthesis (Pulz and Gross, 2004).

A number of microalgae species are able to assimilate CO2 from soluble carbonates such as sodium carbonate (Na2CO3)and sodium bicarbonate (NaHCO3). Due to the high salt content and resulting high pH of the medium, it is easier to control invasive species since only a very small number of algae can grow in the extreme conditions (Colman and Rotatore, 1995;

Emma et al., 2000; Wang et. al., 2008). The selection of suitable microalgae strains for CO2

bio-mitigation has significant effect on efficacy and cost competitiveness of the bio-mitigation process. The desirable characteristics for high CO2 fixation include: high growth and CO2

utilisation rates; high tolerance of trace constituents of flue gases such as SOx and NOx;

possibility for valuable by-products and co-products, e.g. biodiesel and biomass for solid fuels;

ease of harvesting associated with spontaneous settling or bio-flocculation characteristics;

high water temperature tolerance to minimize cost of cooling exhaust flue gases; be able to

use the strain in conjunction with wastewater treatment.

For example, de Morais and Costa (2007a) using Spirulina sp., obtained a maximum daily CO2 biofixation of 53.29% for 6% (v/v) CO2 and 45.61% for 12% (v/v) CO2 in the injected flue gas, with the highest mean fixation rate being 37.9% for 6% (v/v) CO2. With Scenedesmus obliquus, de Morais and Costa achieved biofixation rates of 28.08% and 13.56%

for 6% (v/v) and 12% (v/v) CO2, respectively.

Chang and Yang (2003) found that certain species of Chlorella could grow in an atmosphere containing CO2 up to 40% (v/v). When comparing Botryococcus braunii, Chlorella vulgaris and Scenedesmus sp. under flue gas conditions, Yoo et al. (2010) found Scenedesmus sp. to be the most suitable for CO2 mitigation due to high rates of biomass production (0.218 g L^-1 d^-1). B. braunii and Scenedesmus sp. were found to grow better using flue gas as compared to air enhanced with CO₂. This is similar with an earlier study by Brown (1996) who found that microalgae can tolerant with flue gas very well.

In contrast, CO₂ capture from flue gas emissions from power plants that burn fossil fuels achieves better recovery due to the higher CO₂ concentration of up to 20% (Bilanovic et al., 2009), and adaptability of this process for both photobioreactor and raceway pond systems for microalgae production (Brennan and Owende, 2010). Flue gases from power plants are responsible for more than 7% of the total world CO₂ emissions from energy use (Kadam, 1997). Also, industrial exhaust gases contains up to 15% CO2 (Maeda et al., 1995; Kadam, 2001), providing a CO2-rich source for microalgae cultivation and a potentially more efficient route for CO2 bio-fixation. Table 3 summarized the productivity of biomass grown outdoors in the various photobioreactors. These outdoor cultivation showed the potential of microalgal cultivation for on-site bioremediation of CO2 from flue gas.

In order to have an optimal yield, these algae need to have CO2 in large quantities in the basins or bioreactors where they grow. Thus, the photobioreactors need to be coupled with traditional electricity-producing thermal power centers that produce CO2 at an average rate of 13% of the total flue gas emissions. The CO2 is put into the photobioreactor and assimilated by the algae. Outdoor microalgal culture coupled with flue gas aeration is with economic value and potential strategy for large-scale microalgal cultivation.

GreenFuel Technologies, one of the earliest, best funded and most publicized algae companies was a startup that developed a process of growing algae using emissions from fossil fuel, mainly to produce biofuel from algae. A beta emission reduction system was

installed at an MIT cogeneration facility in 2004 and after performing beyond expectations was moved to a larger power plant in fall 2005. Pilot units were tested at power plants in Arizona, Massachusetts and New York. Although the algal biomass produced by the process consists of proteins, lipids and carbohydrates which could be used to produce a variety of products, GreenFuel Technologies seems to be focusing on biofuel products. GreenFuel's large scale algae to biofuel process at the Arizona Public Service Redhawk power facility won the 2006 Platts Emissions Energy Project of the Year Award.

In 2007, the company had to shut down its third-generation bioreactor facility in Arizona after the plant produced more algae than the company’s equipment could handle. At the same time, the company learned that its algae harvesting system would cost twice as much as expected. Though GreenFuel Technologies finally shut down operations on May 13, 2009, GreenFuel Technologies was the frontier of industrialized process of microalgal cultivation and microalgal bioremediation. At present time, there are many microalgal industries for microalgal cultivation process and they also focus on microalgal bioenergy. Table 4 list the main international companies that using microalgal cultivation for CO₂ reduction and bioerergy development.

1.9 Potential of Biodiesel Produced from Microalgae

There are four important potential benefits of algae biomass cultivation that other sources don’t have. First, algae biomass can be produced at extremely high volumes, and this biomass can yield a much higher percentage of oil than other sources. Second, algal oil has limited market competition. Third, algae can be cultivated on marginal land, fresh water, or sea water. Fourth, innovations to algae production allow it to become more productive while consuming resources that would otherwise be considered waste (Campbell, 2008). Biodiesel derived from oil crops is a potential renewable and carbon-neutral alternative to petroleum fuels. Unfortunately, biodiesel from oil crops, waste cooking oil, and animal fat cannot realistically satisfy even a small fraction of the existing demand for transport fuels.

Microalgae appear to be the only source of renewable biodiesel that is capable of meeting the global demand for transport fuels.

Like plants, microalgae use sunlight to produce oils, but they do so more efficiently than

Like plants, microalgae use sunlight to produce oils, but they do so more efficiently than