Making Biodiesel From Algae

Extraction Processes Vary for This Green Fuel

Algae in lab
U.S. Dept. of Energy

Algae is easy to produce and requires less land to do so than many other plant sources commonly used in the making of fuels, making it an attractive candidate for full-scale biodiesel production. In addition, with a composition containing about half lipid oils, algae appear to be a rich resource as a biofuel feedstock.

So how do you move from tiny green plant to biodiesel? There are several things to know about algae biofuel production.

The following questions and answers help to outline the process.

Algae Contain a Lot of Oil--How Is It Extracted?

Not surprisingly, there are numerous ways to remove the lipids, or oils, from the walls of algae cells. But you may be surprised to learn that none of them are particularly earth-shaking methods. For example, ever hear of an olive press? One of the ways for extracting oil from algae works very much like the technique used in an oil press. This is the simplest, yet most common, method for extracting oil from algae and yields about 75 percent of the total available oil from the algae plant.

Another common method is the hexane solvent method. When combined with the oil press method, this step can yield up to 95 percent of available oil from algae. It utilizes a two-step process. The first is to utilize the oil press method. But then, instead of stopping there, the leftover algae is then mixed with hexane, filtered and cleaned to remove all traces of chemical in the oil.

Used less frequently, the supercritical fluid method can extract up to 100 percent of available oil from the algae. Carbon dioxide is pressurized and heated to change its composition into both a liquid as well as a gas. It is then mixed with the algae, which turns the algae completely into oil. Although it can yield 100 percent of available oil, the plentiful supply of algae as well as the additional equipment and work required, make this one of the least popular options.

More diversified than the extraction processes are the methods used for growing the algae so that it will yield the most oil.

How Is Algae Grown for the Production of Biodiesel?

Unlike extraction methods, which are practically universal, growing algae for biodiesel varies greatly in process and method used. While it is possible to identify three primary ways to grow algae, biodiesel manufacturers have worked hard to tweak these processes and make them their own in the quest for perfecting the algae growing process.

Open-Pond Growing

One of the easiest processes to understand is known as open-pond growing. This is also the most natural way to grow algae for the purpose of biodiesel production. Just as its name would imply, in this method, algae is grown on open ponds, particularly in very warm and sunny parts of the globe, with the hope of maximizing production. This is the simplest form of production, but not surprisingly, it also has some serious drawbacks. In order to truly maximize algae production using this method, water temperature needs to be controlled which can prove very difficult. In addition, this method is more dependent on weather than others, another variable that is impossible to control.

Vertical Growth

Another method for growing algae is a vertical growth or closed loop production system. This process actually came about as biofuel companies sought to produce algae quicker and more efficiently than what was possible utilizing open pond growth. Vertical growing places algae in clear, plastic bags which allows them to be exposed to sunlight on more than just one side. These bags are stacked high and protected from the elements with a cover. While that extra sun may seem trivial, in reality, the clear plastic bag provides just enough exposure to sunlight to increase the rate of the algae production. Obviously, the greater the algae production, the greater the potential amount of oil that will later be extracted. And unlike the open pond method where algae is exposed to contamination, the vertical growth method isolates algae from this concern.

Closed-Tank Bioreactor Plants

A third method of extraction that biodiesel companies are continuing to perfect is the construction of algae closed-tank bioreactor plants to further increase already-high oil production. In this method, algae isn't grown outside. Instead, indoor plants are built with large, round drums that are able to grow algae under near perfect conditions. Within these barrels, the algae can be manipulated into growing at maximum levels--even to the point they can be harvested every day. This method, understandably, results in a very high output of algae and oil for biodiesel. Some companies are locating their closed bioreactor plants near energy plants so that extra carbon dioxide can actually be recycled rather than polluting the air.

Biodiesel manufacturers continue to hone the closed container and closed-pond processes, with some developing a variation known as fermentation. In this method, algae is cultivated in closed containers where it is "fed" sugar to spur growth. Following this process is attractive to growers since it provides complete control over an environment. A lesser advantage is that this method is not reliant upon weather or similar climatic conditions to be viable. However, this process has researchers considering sustainable methods for obtaining enough sugar to maximize algae production.