Biofuels are fermented alcohol fuels made from biological resources such as plants and animals. Since biofuels are made from biological resources, they are considered a form of renewable energy. Biofuels do not increase the amount of carbon dioxide in the atmosphere when used as fuel because they absorb carbon dioxide from the atmosphere through photosynthesis during the growth process of the plants used as raw materials. For these reasons, biofuels are attracting attention as an alternative to fossil fuels such as petroleum, which is likely to be depleted in the future. As an example of their use, biofuels are blended with gasoline in automobiles.
On the other hand, biofuels also have a major challenge: they compete with food. Bioethanol, a typical example of biofuel, is made from sugarcane and corn grains. With many people in the world starving due to food shortages, the use of food resources for biofuels is questionable. Therefore, there is a need to use resources other than food crops.
Algae are being actively researched because they produce a higher amount of oil per crop area than other crops and can be used on land that would normally be unusable for agricultural purposes. Algae are photosynthetic organisms that do not live on the ground; a familiar example is seaweed.
There are two methods of cultivating algae: outdoor culture tanks and sealed culture tanks. Outdoor cultivation is relatively simple and cost-effective. However, the problem is that the production of the target algae is inhibited by the presence of organisms other than the target algae. On the other hand, closed-type culture allows adjustment of the culture environment, making it possible to culture at high concentrations by setting appropriate conditions. Cultivation at high concentrations encourages algae to actively photosynthesize. Therefore, it is important to supply carbon dioxide to the culture tank efficiently.
The bubbling method is a common method of supplying carbon dioxide. However, this method is inefficient because most of the carbon dioxide supplied is not dissolved in the culture medium and is released into the atmosphere.
The use of hollow fiber membranes as an alternative to the bubbling method has been reported to provide carbon dioxide efficiently1). In this report, a silicon module (NAGASEP) was used as the gas supply membrane, and since NAGASEP has good permeability for carbon dioxide gas, air (oxygen concentration 21%, carbon dioxide concentration 450 ppm) was supplied to the module and the permeation side was depressurized, The permeated air is then concentrated to 30% oxygen and 2,000 ppm carbon dioxide. Thus, NAGASEP can supply a high concentration of carbon dioxide gas to the culture medium and promote photosynthesis.
