Photobioreactors

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Photosynthesis has long been recognized as a means, at least in theory, to sequester anthropogenic carbon dioxide. There has been relatively little research aimed at developing the technology to produce a gaseous combustion effluent that can be used for photosynthetic carbon sequestration. 

1.  Description

2.  Why

3.  How

4.  Future Trends

5.  Related Links

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Description

The photosynthetic reaction process by plants is too slow to significantly offset the point source emissions of CO2 within a localized area. Aquatic microalgae have been identified as fast growing species whose carbon fixing rates are higher than those of land-based plants by one order of magnitude.The vision is a viable strategy for carbon sequestration based on photosynthetic microalgae. CO2 from the fossil fuel combustion system and nutrients are added to a photobioreactor where microalgae photosynthetically convert the CO2 into compounds for high commercial values or mineralized carbon for sequestration.The advantages of the proposed process include the following.

(1) High purity CO2 gas is not required for algae culture. It is possible that flue gas containing 2~5% CO2 can be fed directly to the photobioreactor. This will simplify CO2 separation from flue gas significantly.

(2) Some combustion products such as NOx or SOx can be effectively used as nutrients for microalgae. This could simplify flue gas scrubbing for the combustion system.

(3) Microalgae culturing yields high value commercial products that could offset the capital and the operation costs of the process. Products of the proposed process are:

  • mineralized carbon for stable sequestration;

  • compounds of high commercial value. By selecting appropriate algae species, either one or both can be produced.

Why

The cultivation of microalgae provides excellent perspectives for renewable energy production and as a source of ‘green’ products. Algae cultivation can thus contribute substantially to a reduction of CO2 emissions.These systems could be applied to reduce greenhouse gas emissions by three main mechanisms:

1. The methane rich (typically >85%) biogas produced in the initial facultative ponds can be collected using submerged gas collectors, thereby reducing the atmospheric emission of this greenhouse gas.

2. The gas can then be used to generate power to mix the ponds and run the operating equipment, thus avoiding the fossil CO2 emissions from the power consumed in conventional waste treatment.

3. Harvesting the algal biomass and its conversion to biofuels would replace additional fossil fuels. The algal cultures in the high rate ponds are generally CO2-limited, and supplemental CO2 would greatly enhance algal production and biomass and thus, biofuels production.

How

Emissions of carbon dioxide are predicted to increase this century1 leading to increases in the concentrations of carbon dioxide in the atmosphere. While there is still much debate on the effects of increased CO2 levels on global climate, many scientists agree that the projected increases could have a profound effect on the environment.Carbon sequestration, capturing and storing carbon emitted from the global energy system, could be a major tool for reducing atmospheric CO2 emissions from fossil fuel usage.

  • Most of the anthropogenic emissions of carbon dioxide result from the combustion of fossil fuels for energy production.

  • It is the increased demand for energy which underlies the projected increase in CO2 emissions.

  • Meeting this demand without huge increases in CO2 emissions requires more than merely increasing the efficiency of energy production.

Micro-algae are microscopic, single-celled plants, growing in aqueous environment. For growth algae make use of sunlight as energy source and simple inorganic nutrients, predominantly CO2, soluble nitrogen components and phosphates.Characteristics of algae cultivation are:

  • The areal productivity is 2 to 5 fold higher as compared with traditional agricultural crops and fast growing ‘energy crops’ such as willow and Miscanthus.

  • Lower quality water can be used for growing algae, e.g. the effluent of biological waste water treatment facilities. Algae effectively remove nitrogen and phophate from these streams, which leads to a reduction of water treatment costs. After separation of the algae (and final conditioning) the purified water can be reused for industrial purposes.

  • Algal systems can remove CO2 (and NOx) from flue gases. The flue gases from power plants can be directed through the algal bioreactor. CO2 is taken up by the algae and directly recycled in the form of biomass and derived products.

  • Many algal species produce valuable products, such as colorants, polyunsaturated fatty acids and bioactive compounds. These ‘fine chemicals’ are applicable as a natural ingredient in food products, pharmaceuticals, food supplements and personal care products.

After extraction of these valuable compounds the remaining biomass (approx. 80%) can be used for production of ‘green’ electricity and heat. Alternatively, microalgae can be used for the production of methylesterfuel (‘bio-diesel’). The CO2 content of flue gas from boilers used for power generation ranges from 7 vol% to 15 vol%. Stationary diesel combustors and gas turbines fired with natural gas, also used for power generation, use much higher amounts of excess air and have, therefore, much lower CO2 content, on the order of 3 vol%.

Future Trends

In the United States coal and natural gas are the primary fuels used for power generation, although fuel oil is important in some regions. Conventional boilers (as opposed to gas turbine combustors) employ modest amounts of excess air for combustion.Microalgae can produce high-value pharmaceuticals, fine chemicals, and commodities. In these markets, microalgal carbon can produce revenues of order $100,000 per kg C. These markets are currently estimated at >$5 billion per year, and projected to grow to >$50 billion per year within the next 10-15 years. Revenues can offset carbon sequestration costs. 

Keywords

Biofixation, photobioreactors, Algae as bio-fuel, photosynthetic reaction

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