Microalgae

MICROALGAE

Microalgae are aquatic micro-organisms. Featuring of rapid growth, they have the particularity to produce industrially useful substances. However, their potential is still relatively unexplored: on a million species estimated; only about 30,000 have been studied.
Already used in the food, cosmetics and fertilizers, microalgae are considered as an alternative way to traditional fuels and chemical derivatives of biological origin.
Indeed, some species have the capacity to produce compounds with an energetic potential like the lipids (source of biodiesel) or starch (source of bioethanol). Microalgae are part of so-called third generation biofuels.
The third generation biofuels are different from the agrofuels, which derived from crops traditionally, intended for food and from the 2nd generation biofuels produced from non food plant sources such as wood and plant waste.
Microalgae can also be involved in the production of two other types of biofuels: hydrogen and biogas.
Technical or scientific operation:
Most microalgae convert solar energy using CO2 and water to produce oxygen and algal biomass (organic matter) by a reaction called photosynthesis. Microalgae are characterised by a very high photosynthetic efficiency (ratio of incident light energy and energy stored in the plant).

Cultures of microalgae:
Previously selected, the microalgae are cultured in a controlled manner in order to obtain large quantities of algal biomass.
Usually, the seawater enriched in nutrients (such as nitrates and phosphates) is used for growth of these micro-organisms. However, the use of fresh and brackish water (from lakes or rivers) is also possible.
The main methods of production are:

• Basins open air, inexpensive to operate. However, these open systems have their affected productivity by microbial contamination and water loss by evaporation.

• Photo-bioreactors (transparent tubes forming a closed system). Cost is higher, but it's compensated by higher productivity.

Basin open air

Photo-bioreactor

Applications :

Biodiesel production:

• Grown under conditions known as «stress» (e.g. nitrates deficiency or a sudden increase in light intensity), some species significantly increase production of lipids that can reach 80 % of their dry weight. However, these conditions of high productivity cannot be maintained over long periods. Indeed, they lead to a cessation of growth and consumption of fat reserves produced. The optimization of lipid productivity therefore involves an alternation between growth (without deficiency) and production of oil (with stress which slows growth).
• Microalgae are then harvested and the oil is extracted by different methods (centrifugation, solvent treatment, thermal lysis, etc.) to be converted into biodiesel. Conventional techniques of transesterification, developed for vegetal oils, may be applied: the principle is to react the algal oil with methanol or ethanol.

Bioethanol production:

• The researches into algo-fuels production are mainly oriented biodiesel. However, these micro-organisms also produce carbohydrates (such as glucose or starch) capable of producing another biofuel, ethanol. It is produced during anaerobic fermentation (without oxygen) in absence of light, starting from the starch. The resulting alcohol is then concentrated and hydrated to obtain bioethanol.

Biogas production:

• This is the most accomplished sector from all the productions of biofuels. Algal biomass, concentrated and wet, turns out to be particularly suitable for methanisation process. After fermentation in anaerobic conditions (without oxygen) in a heated digester, it generates a biogas consisting of 70 to 80% methane. This can be used for the production of heat and electricity or directly injected into a gas network.
• An association between biomass production, CO2 capture and biogas production is currently being investigated. The CO2 generated by burning biogas, may be recycled directly for producing micro-algal biomass, the latter being necessary for the photosynthesis reaction.

Biological hydrogen production:

• Parallel to the production of biomass from CO2, microalgae are also capable of supplying hydrogen gas under anaerobic (oxygen-free). However, this phenomenon is limited because of photosynthetic reaction which produces oxygen, inhibits hydrogen synthesis. At present, the conversion of light energy into hydrogen gas is insufficient for this to be profitable. To improve this performance, several approaches have been developed in laboratory, including the genetic modifications.

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