In order to extract the oil found in the algae cell walls, known as lipids, a separation process must take place. There are three conventional processes below which extract different amounts of oil from the algae, although achieving a higher extraction comes at additional costs due to more equipment being required.
The oil can be extracted from algae using a mechanical press, as seen in figure 1. This process—which forces the algal paste into a nozzle that removes oil and expels much drier biomass—can extract up to 70% of the algal oil, but generally requires a dry algae feedstock. Dry algae is the product of dewatering. This process is commonly combined with the hexane solvent method.
Oil from the algal biomass dissolves into the hexane solvent; a six-carbon chain molecule. Then the oil is cleaned through distillation, and the solvent is recycled through further processes. Up to 95% of the oils can be extracted using hexane solvents at low financial and energy costs, although there are some concerns with the health and safety effects of a caustic solvent being used and recycled throughout the process.
Supercritical fluids in general are very high temperature, high pressure fluids that are not clearly liquid or gas. Superfluid carbon dioxide (see figure 2) can be used to rupture algal cells and release oil. Up to 100% of the oil can be extracted, but the equipment and energy costs are very high, which limits commercial viability and interest in this method.
There are many other methods for extracting oil from the algae, but as algae biofuels are not currently being produced commercially, many of these are in research and testing phases. As a result, they are not yet scalable to sizes necessary for commercial biofuel production.
Once the oil has been extracted it must be refined to become a diesel fuel as it is too viscous after the separation stage. Transesterification is the most common method, although there are less used methods such as pyrolysis, micro emulsion, blending and others. In the process of transesterification, methanol and a basic solution are mixed and reacted with the oil. This produces biodiesel and glycerin byproducts. The biodiesel is then washed and filtered before it is ready for public use.
After the oil is removed from the algae there is a lot of solid material, known as biomass. This leftover biomass, can be used to create another algae based fuel product, bioethanol (which is ethanol from biomass). The carbohydrate content of the biomass can be converted into sugars. This can be accomplished through fermentation with yeast, giving off carbon dioxide, which can be recycled back into the growing process of the algae. Then the sugars continue to ferment into ethanol. Also, in the case where the algae still contains the original oil in the biomass, decay can be initiated in order to rupture cell walls, releasing the carbohydrates, then followed by fermenting (figure 3).