Photobioreactor

In the period of global climate warming and of soaring price of petroleum fuel, biofuels is a potential renewable and clean alternative. Traditionally, biofuels come from two sources: Biodiesel from oil plants such as sunflower or rapeseed, and alcohol from the fermentation of sugars from beets, wheat, or corn. Unfortunately, these fuels can not meet even a small fraction of existing transport demand since our planet can not supply so large cultivation land for these crops. Like crop plants, microalgae use sunlight and CO2 to produce oils but they work more efficiently than crop plants. Some microalgae are particularly rich in oils and its oil yield per hectare is considerably higher than that of sunflower or rapeseed. Studies on microalgal biodiesel showed that 1 to 3% of the total U.S. cropping land would sufficient to produce algal fuels to meet 50% of transport fuel needs. Microalgae appear to be the only source of biofuel to completely replace petroleum fuel in the future.

Oil content of some microalgae

Y. Chisti / Biotechnology Advances 25 (2007).

Microalgae are single cell, sunlight-driven factories that use sunlight to convert carbon dioxide to potential biofuels, foods, and high-value molecules. Microalgae have been used as human food, cosmetics and medicine for thousand years. Besides that, microalgae also were used to CO2 mitigation , the bioremediation of polluted river and to produce biofertilizer. Unlike Terrestrial plants, microalgae grow extremely rapidly and commonly double their biomass within 24 hours. Microalgae with high oil content are desired for producing biofuels.

Raceway ponds and tubular photobioreactor are two practicable large-scale production systems in present time. A raceway pond is made of a closed loop recirculation channel. The flow is mixed and circulated by paddlewheels which operates all the time to prevent algae sedimentation. Raceways are low-cost and easy to operate. But its temperature fluctuates with sunlight, and water evaporation is not controlled. Contamination by microorganisms and unwanted algae is also easy. The productivity of raceway systems is often lower than photobioreators.

Unlike open raceway systems, photobioreators consist of an array of transparent tubes. The tubular array, the solar collector, can be oriented to maximize sunlight capture. The temperature is controlled by green house facilities or air conditioning equipments. Microalgal broth is circulated from a reservoir to the solar collector and then back to the reservoir. Therefore, photobioreactors permit single-species culture of microalgae for prolonged durations without contamination. The biomass productivity of photobioreators is greater than that of raceway ponds, and the efficiency of CO2 trap and NOx uptake is better too. Photobioreactor is expensive. But the typical biomass concentration in photobioreators is nearly 30 times that in raceway ponds. Thus, in comparison with raceway broth, much smaller volume of photobioreactor broth needs to be processed to get the same quantity of biomass. Since photobioreactors is a close system, CO2 and NOx emitted by power plants could be completely absorbed. Its application on CO2 mitigation at the industrial level is under way in many industrial countries.

The future studies will be concentrated on:

---increase algal photosynthetic efficiency and biomass growth rate;

---increase oil content in algal cells;

---improve the stress tolerance of microalgae;

---develop new photobioreactor system to decrease the cost of algal cultivation, biomass harvest and        biorefinery processes.

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