Algae, like many other plants, produce oil completely naturally, as a strategy for storing energy. When plants and algae are exposed to sunlight, water and CO2, they begin to photosynthesize, transforming carbon dioxide into sugar. So far, so good.
Land plants must use part of the sugar they produce to form cellulose and build cell walls to construct leaves, stems, roots, and reproductive structures. The remaining starch and lipids are stored as a food source. Since microalgae don’t need to form stems, leaves, or reproductive structures, almost all the sugar they produce is metabolized into lipids (oils). In fact, as much as 75% of some types of algae masses are composed of oil. Algae also grows much faster than any other plant, meaning that algae can produce up to 30x more oil than common biodiesel sources like soybeans, oil palms, and rapeseed (canola). Some studies have even suggested that optimized algae production methods can yield up to 100x more oil per acre.
Since algae oils can be used for many different commercial and industrial purposes, the variety of algae to be cultivated and the extraction method used both depend on the ultimate use.
In the past decades, improvements in algae strains, feeding, CO2 efficiency and harvesting have significantly lowered the cost of oil extraction from macroalgae’s and yet improvements are still necessary to bring the production cost of biofuels down to levels where they can reasonably be expected to replace fossil fuels.
When algae are harvested, the valuable oil must be extracted from within the algal cells themselves. This is accomplished through physical, chemical, or some combination of processes. In either case, the vegetative material that remains is called biomass.
Physical extraction
As you might imagine, the simplest method of extracting oil from the cells of algae is crushing. Since lipids do not evaporate, it’s possible to completely dry algae without affecting its oil content. After drying, lipids are squeezed out of the algae as it is passed through an oil press. While this is a simple process and closely mirrors oil extraction methods for oilseed crops, algae presents some unique challenges, such as small cell size and thick cell walls, so oil presses may include the use of screws, expellers and pistons, depending on which strains of algae are being processed. Even so, additional steps may be necessary when working on a commercial scale.
An alternative to crushing algae is to employ an effect known as osmotic shock. In this case, a sudden drop in external osmotic pressure is induced, which causes the algal cells to rupture and release their oil, which can then be separated from the remaining biomass. This can work well when applied to suitable strains of algae, but it often requires chemical solvents to be used to maximize effectiveness.
Ultrasonic extraction is another method used in lipid recovery, usually in conjunction with chemical solvents. In this process, ultrasonic waves are induced in a solvent solution that contains algal cells. The waves create cavitation bubbles, which in turn trigger shock waves when they collapse near cell walls, thereby breaking the cell walls and releasing the lipids.
Solvents
Solvents are typically used to weaken cell walls, and in the case of algae, they can make physical attacks more effective. Hexane, an organic compound, is often used in the food industry; chemical solvents like benzene and ether are also effective. Still, there are downsides to using solvents in these processes. Exposure to vapors and skin contact can cause serious health problems to workers, many chemicals themselves are explosion hazards, and some are known carcinogens. Disposal of used solvents and potential environmental damage represent long term management issues that are somewhat antithetical to the goal of establishing an environmentally friendly, sustainable fuel source.
Another type of extraction using chemical solvents is called the Soxhlet extraction. This involves repeated washing with an organic solvent such as hexane. One important advantage to this technique is that the solvent can be reused.
Enzymes can be used with water to degrade cell walls, but this method is considerably more expensive than other options.
CO2 can also be used as a solvent when in a supercritical state. A supercritical fluid is a substance held between a liquid and a gas phase which exhibits properties of both. There are some significant advantages to using supercritical CO2 extraction, as it is readily available, inexpensive, non-toxic, and separation of solvent and extract is an easy process. One disadvantage to this method is that initial capital costs for equipment are higher than other options, but in the long term, costs are generally lower.
