Based on their size algae are divided in to two broad groups, these are Macroalgae and Microalgae. Macroalgae, which commonly called sea weeds are multi cellular algae; some of its species could grow up to 50 meters in diameters. On the other hand Microalgae are microscopic algae. Depending on the particular species both macroalgae and microalgae can grow in fresh water, sea water and seawages. Difference in size is not the only difference between these two groups, they also differ in the form of the compound they store in their cells. Macroalgae mainly store carbohydrates in their cells for energy source, but microalgae store mainly Triacylglycerol in their cells for energy source. Triacylglycerol is the major feedstock in biodiesel production. Therefore for their higher Triacylglycerol content in their cells, microalgae are considered the potential source for biodiesel production.
Microalgae also have other qualities, to be named the number one choice for biodiesel production:
- They have very fast rate of growth and reproduction than macroalgae and other oil crops. They can be ready to be harvested with in just a few days.
- They are microscopic and lives in aggregates so they allow different designs of photobioreacors and ponds.
- They can grow easily on seawages and utilizes their nutrients from the seawage water.
- Two types of products can be produced from microalgae these are ethanol and biodiesel.
- Carbon dioxide from coal power plants and flue gases can be utilised. Flue gases contain in addition to carbon dioxide, hydrogen nitride, sulphides, sulphur dioxide and other gases. Microalgae utilises the CO2 for photosynthesis, and the other gases as nutrients.
- Microalgae purifies waste water which comes out from factories and farms such as fertilizers, iron, manganese, zinc, chromium, nickel, cadmium and cobalt. One famous microalgae used for such purpose is Chlorella vulgaris which purifies wastewater from metals.
- And most of all microalgae have very high Triacylglycerol content which could reach up to 54% in dry weight basis.
Microalgae are classified in to four groups these are Diatoms, Green algae, Golden brown algae and Blue green algae. In total there are more than 200,000 species of microalgae. Although the number of spices of microalgae is so vast, only a few are used for biodiesel purpose. Those microalgae produced for biodiesel production are highly oleogenic producing more than 40% of oil per dry weight basis. Some of the species of microalgae which are growen for biodiesel purpose are Nannochloropsis spp (54%), Botryococcus spp (54%), Nitzschia spp (47%), Botryococcus braunii, Dunaliella salina, Spirulina spp, Dunaliella spp, Scenedesmus spp and Chlorella spp.
For optimum production of biodiesel from microalgae, artificial creation of stress such as nitrogen starvation and environmental stresses such as very high temperature and light intensity will tremendously increase the quantity and quality of the oil produced. And it is the number one strategy to increase the oil yield.
There are two major sources of CO2 these are, compressed CO2 and CO2 from power plants. Compressed CO2 is very expensive to use for large scale biodiesel production. And Injection of CO2 directly from power plants to photobioreactors and ponds have two problems, first it has very high temperature which many species of microalgae can't resist except those of Chlorella spp, and second the gas mixture in addition to CO2 contains other gases such as NO2 and SO2 in very high concentrations which is toxic for the algae. To avoid the high temprature problem the gases need to cool down and reduced to 2% concentration level before injection.
